Oh yeah, and advantage of the bridge-wall Super-Floodgate method is also that you can build it all at once, regardless of width (10-wide maximum per-section, however), unlike floodgates that are 1-by-1. Meaning, provided you can slow down a wider river with the same setup extended, you can apply these to wider rivers overall. Just hope you have the material to get this all taken care of. Someone up to testing this method?
done... super-floodgate works 
Only one "pumpstack" (drain upstream) was necessary as the river drained sooo fast.
Took 2,5 seasons after embark, 47 pumpstacks, 6 water wheels and a lot of wasted time to set up 
The important thing is that the proof of concept works. So dwarves can traverse the dried areas (or at least 3/7s (gaining swimming skill while at it)) no problem now without panicking? Cool.
Nice to know. Now the next experiment is that offshore rigging modular drain build... thing that I proposed in an earlier post. Now that may waste quite a bit more time; but if that works using this proof of concept's methods as a method of application, then the offshore oil rig fort idea (without magma/obsidian-casting) can be a possible megaproject.
A good Z-level amount of water to drain, I recommend to at least prove the concept may work is maybe 3Z deep? You can try deeper than that if you want, but 3Z should be enough depth to see if a 10x10 (or 8x8-9x9 minus the S-FGs making up the perimeter) area can be drained and used as a sea floor work shaft being built from top-to-bottom (sea level to below). To further prove the concept, make a sort of airlock on the surface if waves crash atop it to prevent water from going in (even a floor hatch would do), send a few brave dorfs across the walkway while the waves crash, and go into the dwarfmade shaft and build/dig down the hole below it. I recommend you know how to work with or shut off aquifers before doing this challenge/step.
I haven't come up with a good enough rapid-deconstruct method that doesn't sacrifice some poor saps to the detonation of them, or getting drowned by the freeing of them; unless you feel like saving the game, shutting off cave-ins (unless a DFHack plugin can be made to trigger it mid-game), reloading it, and having a dwarf or few that can brave the waters deconstruct the bottom block, and rush to safety (I would suggest burrows, and assigning each highly skilled dwarf with equal skill levels to each portion, and then deconstructing them simultaneously, hope they make it back to base (water should be flowing at a slower pace, seeing as water traveling diagonally flows slower by pressure re-regulation, somehow), repeat the first steps except turn on cave-ins, and watch the water flood the areas and the pumps and bridges and etc. fall apart and a perimeter of discarded materials lie where they were.
With a setup like that, with a central shaft, an offshore floating water fortress should be possible. Bonus points if you can make it look like a hovercraft; more points if you can hide it's oceanic support structure in plain sight. Of course, if you have a [BUILDING_DESTROYER] reach the support beam under the hovercraft, well... goodbye hovercraft. Overall, I think a central column of up/down stairs could work like an ocean lab ladder or something, where a hovercraft would be concerned. Supports the ship, and hides in plain sight, while permitting it's builders to bail out before the flooding of the support's area. I suggest having a "Kill-em-all" switch linked to the support that rests atop it, and it remains the only link to the ship and the waters below. Think of it like shutting off the fans that keep air inside the skirt of the ship.
The "ladder" can also be used to support a classic boat in the water for anyone that wants to relive the Pirates of the Fondled Waters succession/community fort. Just make sure the boat is deep enough in the water for a believable water displacement for the size of ship in question (like, at least a deck or 2 into the waters themself).
EDIT:
Alternatively, you can always freeze the water, and take care of deconstruction and cleanup, and then warm it back up (works best in cold climates, and then shutting off temperature when the water freezes over.
EDIT EDIT:
I suppose an easier method to apply for offshore building would be to make this dam from shore to sea fending water away from 2 directions (or essentially making a "Moses Bridge"), and then establish a perimeter and build downward until you have a dry area, and then procedurally deconstruct it on the way back to shore when you finished. But I still prefer caving in the old sea walls that made up the perimeter for a proper flooding that renders the offshore structure officially independent from the mainland. The old method also only really requires deconstructing a single tile that links to the main shaft and the mainland that connects to the drainage perimeter system. My first idea by building it instead of Moseseying to the designated "Offshore rig" area should also take less time and resources to produce and execute. Plus, doing a modular 10x10 sections at a time should also save on resources, time, and logistics to work on it and for the computer to process FPS-wise, since the area to process is small sections at a time, instead of a giant chunk all at once, which would take ages per-Z-Level. Both in-game and realtime. This should theoretically be cheaper time-wise in construction and processing of the water drainage.
EDIT EDIT EDIT:
Think of it as an improvement of
this idea. I'm hoping my tweaking should render the process easier and cheaper to do, and can be entirely (and entirely built) independent from the land (being 4 walls made, aside from the walkway to reach it, and not 3, and using the breakers like I mentioned before as a way of anchoring the drawbridges on the build downward), unlike this one which is directly extending a fort from the land itself. Plus, instead of wall sections being established, it's these Super Floodgates instead, which can be started/built from the point closer to a safe spot; and although it's open when it's laid down at first, it stops everything immediately when it goes up. Think of it like a slatted vent system that when closed, is airtight.
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Topic: GUIDE: How to dam/drain a 4 tile wide river/brook without digging (Read 6488 times)