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[Jigowah]

I'm considering using the nearby brook to create some kind of water defense flood/cannon outside my exterior gates.

My current ideas are:
Drowning airlock (easy to make, keeps the loot)
Artificial waterfall that could push enemies into drains and then into the caverns (lose the loot, enjoy the chaos)
Pressurized water cannon (i have never weaponized water pressure before so this design would be new to me)
Ice Cube Freezer (its too hot for this to work at my embark)

What other kinds of water defenses might be good? 

[Thisfox]

Gobbo McGreenskin cancels attack fortress: Interrupted by carp.

[NW_Kohaku]

Generally speaking, magma-drowning chambers are preferred, as they automatically incinerate all non-iron loot for you, removing the need to manually sort out the troll wool socks.  It does, however, have the moderate complication of a much longer path from magma to moat outside of a volcanic embark. 

In general, though, the wiki covers this concept. 

What you usually want to do is store the fluid in a reservoir above the entrance. 

One method has several downramps on either end to prevent water overflow (this also means you don't have to seal off the chamber) and use water pressure to sweep targets into a drowning trench which seals shut afterwards with a retractable drawbridge, and then build a system of pumping water back up into the reservoir for reuse.  (The drowning trench can also include amphibious pets like crocodiles and glass walls near the dining hall for the dining amusement of your dwarves and the crocodiles.) This method can be used with a seemingly easy-to-path straight secondary entrance to your fortress while the main entrance has lots of twists and turns. (Bonus if it is closer to your prime dining hall.) Make it restricted space to discourage dwarves, put doors up on the inside that are not pet-passable, and set up some pressure plates to trigger the trap.

The other method is to seal off the drowning chamber, and rely upon cross-hatched pumps taking water into/out of a reservoir. (Works best with narrower entrance corridors and a reservoir larger/taller than the drowning chamber.) This lets you seal off your main entrance, and flood it, then reuse it as a safe path for your regular dwarves, but can also be a little more manually-intensive.

[Jigowah]

Very interesting.  For clarity:
Are you saying that ramps prevent all fluid overflow?  Does this also apply when there is fluid pressure?

I would love to use magma, but so far the only magma I have is in a lake at the VERY bottom of the embark.  I'm tempted to make a pump stack, but it seems so tedious that I have yet to find the motivation build it.  Maybe I should suck it up and just do it.

Your drowning pool ideas are sublime, so thank you for that!  I have a brook running under my surface castle wall so water is RIGHT HERE and I might just go with the water options.

Decisions decisions...

[NW_Kohaku]

Very interesting.  For clarity:
Are you saying that ramps prevent all fluid overflow?  Does this also apply when there is fluid pressure?

I would love to use magma, but so far the only magma I have is in a lake at the VERY bottom of the embark.  I'm tempted to make a pump stack, but it seems so tedious that I have yet to find the motivation build it.  Maybe I should suck it up and just do it.

Your drowning pool ideas are sublime, so thank you for that!  I have a brook running under my surface castle wall so water is RIGHT HERE and I might just go with the water options.

Decisions decisions...

Ramps prevent water overflow from pressure if the ramps go at least one z higher than the source.  If you have a pump on the same z-level as the flooding chamber, then up ramps from there prevent flooding outside of that point.  (Keep in mind that pumps are destroyable, and creatures can swim through fortifications that are flooded.  Being 1 z up higher and using grates can be more helpful, but you need taller ramps to compensate.

And yeah, that's where magma normally is.  Remember that you only need magma pumps for really large applications of magma (like flooding the surface), a magma piston or cart-based system using impulse ramps can supply a flooding system that is designed for minimal losses if you have a bit of patience without going the full-on pumpstack (and requisite magma-safe materials and power generators).  I've built one for my most recent fortress, and it's taken a decent chunk of time (about a year and a half) for my miners and engravers to carve out (including a single water reactor to power the pump and under-magma cart rollers), but I can use it for near-surface magma forges, and obisdian-casting portions of my stream so I can work on exploiting the water source in peace.

[Pearofclubs]

I like to put bridges in the room that you lure the goblins to, then drop them into my drowning chamber. Preferably before there's any water in it. You can put some upright spears in the room to speed things up, and that makes a good use for inferior spears your weaponsmith makes.

Then fill it with water and close the bridges up top. The room takes on this lovely red color, and emptying the water allows dwarves to grab the loot. It's more fun to watch than a traditional drowning pit

[Miuramir]

I'm considering using the nearby brook to create some kind of water defense flood/cannon outside my exterior gates.
...
What other kinds of water defenses might be good?

Depending on the situation of your site, variants of "Load minecarts with water, fire it/them at invaders" may or may not be practical.  In some versions you could set up a self-contained system that would basically slam the cart against a fortification to launch the contents downrange; and Water [833] would be treated as a single projectile, doing considerable bludgeoning damage and knockback.  I'm not clear if that still works well; the search term you're probably looking for is "minecart shotgun".  The advantage of this system is that it can be set up to be fully automatic; pull the lever / trip the plate, firing continues until reset. 

Failing that, a possibly simpler and more deadly solution, but one requiring a lot more dwarf-powered resetting, is to just launch the water-filled minecarts at the bad guys directly.  Intermediate re-usability is to have a "doge 'em" trap that sends the water-filled minecarts back and forth or around and around, until they hit an invader trying to path through the death zone. 

With care, you could develop a system that works for water now, but can be upgraded to fire magma if and when you get around to creating a pump stack.  This would let you get the bugs out with a more forgiving fluid, and probably get you results sooner as well. 

[Agent_Irons]

Something that I sometimes do on nonvolcanic embarks is just tunnel down to the magma then back up again for an entrance. Build big walls and chop down trees in the courtyard, if you must.

 The primary design is a tunnel with a low ceiling that enters a little valley. Drawbridges slam shut and then fill it with water. The trick is building such a contraption so that it drains quickly and can't be smashed from the inside, but is big enough to trap invaders even when your dwarves are slow with levers.

[Evil One]

Something that I sometimes do on nonvolcanic embarks is just tunnel down to the magma then back up again for an entrance. Build big walls and chop down trees in the courtyard, if you must.

 The primary design is a tunnel with a low ceiling that enters a little valley. Drawbridges slam shut and then fill it with water. The trick is building such a contraption so that it drains quickly and can't be smashed from the inside, but is big enough to trap invaders even when your dwarves are slow with levers.

For me, I have a huge reservoir about 10 levels above my main entrance chamber usually 7x7, which when the floodgates are opened automatically retracts the main bridge (which is over a huge chasm outside) and seals the fortress (main doors has a series of floodgates protecting them), washing the invaders out of the room into the chasm then down about 10-20z levels, onto a series of floor grates... Yes I have to sort through all the junk, but that means I never run out of stuff to sell to the traders and it's a very effective trap, even if the victims CAN swim they still die and due to the entrance being only 1z level high and 5 across the force of the water stuns them to the point where they can't fly (if they even have the ability).

[Jigowah]

As I consider the drowning room, it actually sounds easier to simply put a huge retractable bridge over a very long fall and skip fluids entirely.  Aside from flyers it seems perfect.

[Pearofclubs]

As I consider the drowning room, it actually sounds easier to simply put a huge retractable bridge over a very long fall and skip fluids entirely.  Aside from flyers it seems perfect.

Well, sure.
Go all feng shui on the bottom with some upright spikes/spears.
Not only does it kill faster, and give that Mortal Kombatish charm, but you can also make use of some lower quality spears from training your smiths.
Plus, if you decide to not use water in this setup, you can then use the shredded goblins as live fire target practice for your marksdwarves. All it takes is a couple fortifications.

 

[NW_Kohaku]

As I consider the drowning room, it actually sounds easier to simply put a huge retractable bridge over a very long fall and skip fluids entirely.  Aside from flyers it seems perfect.

Keep in mind the bridges will not work with a Titan or other large creature on top of it.  Such a bridge is for goblin use, only.

[Eldin00]

Quite a few of said large creatures don't drown either. The takeaway here should be that most defenses you build will have some creatures that can avoid them.

[Miuramir]

As I consider the drowning room, it actually sounds easier to simply put a huge retractable bridge over a very long fall and skip fluids entirely.  Aside from flyers it seems perfect.

Keep in mind the bridges will not work with a Titan or other large creature on top of it.  Such a bridge is for goblin use, only.

We're drifting fairly far off the aquatic defense questions of the OP, but...

True, a small single-element bridge will not operate if the foe is heavy enough.  But if you have a Moria-style support pillar rising up out of the chasm, with a bridge out from either side of the top; when you pull the lever to deconstruct the support, the bridges should become unsupported and collapse, dropping even the largest beast into whatever depths you have arranged. 

If you want to be particularly clever, you can probably arrange it so that you have ready-to-go retracting bridges on the sides, that are designed to stay retracted under normal circumstances.  You can crank them out temporarily to resume traffic while working on re-setting the main bridge after the attack. 

I've been casually considering some chasm-based defense ideas lately; in particular, trying to find interesting options for having a chasm crossing that is semi-routine for everyday dwarf and even caravan use, but under siege becomes a narrow, winding, hazardous walkway covered by ballista fire and marksdwarves behind fortifications.  The intersection of cheap-bolt-resistant, flying / perfect climbing, and building destroyer isn't zero; but filtering down to only those able to even access the inner fortress should lead to pretty good results most of the time. 

Another related thought: rather than building an elaborate pump stack to deliver magma to invaders near the surface, why not delve a much simpler chasm and deliver the invaders to the magma?     A 100+ level fall should render just about anything into a mix of salsa and recoverable metal scrap; add some magma pre-sorting if desired, and haul it right across the hall to your magma smelters. 

[NW_Kohaku]

I realize that it's a limitation that can't be worked around without a more complex trap, but it's worth mentioning it in the forum, rather than letting it become FUN because nobody bothered to tell the OP.

Anyway, the problem with a support-based bridge is that it's single-use, and manually intensive to set up.  As a second- or third-line defense, it makes decent sense, although if you're going to that much trouble, it might as well be a cave-in trap that's significantly more lethal against webbing, syndrome, or metal FBs, which are the only creatures worth going to that much trouble to stop. 

For caravan use, I'm actually moving towards building an "overpass".  An elevated road (or just some ramps up to a small tower) that has pathways to the edge of the map, where I close off access to the trade depot with a drawbridge unless a caravan is in the neighborhood.  This prevents siegers from spawning in the overpass (no access to fort internals) and segregates caravans to likely safe areas.