Check out this super-efficient drowning chamber design. In a nutshell, its benefits are:
1. Fills near-instantly
2. Drains near-instantly (once requested to)
3. Goblinite doesn't get pushed around or disappeared by water flow
4. Safe to operate (you can always leave that part out if you savor the risk of drowning your fort along with the goblins)
The key to success is realizing that water can flow down stairs but doesn't carry items with it when it does. Here it is:
z=2
###############
#...#......B <--- inlet
#...#......####
#...#......#
############
z=1
############
#RRR#R #
#RRR# #
#RRR# #
############
z=0
#######
#>>> ▲#
#>>>###
#>>>#
#+###
z=-1
############
# B▼#▼%% --> drain
# ### #
# ########
#+###
This one is 3x3, but it scales to just about any size. The floor of the drowning chamber is dug out all down stairs, with no connecting up stairs below**. A retracting bridge one level above forms the ceiling. A 2z-tall cistern next to the chamber uses a retracting bridge and drawbridge to form a levered airlock, and a final drawbridge underneath opens the drain. Each level of the cistern holds exactly the volume of the drowning chamber and its underfloor; the bottom level must be the same size as the top one.
The chamber starts with the floor bridge extended, the ceiling bridge retracted, the drain closed, and the airlock open to the inlet (allowing the cistern to fill).
To use:
1. Extend the ceiling to trap the victims who have fallen inside.
2. Toggle the airlock. The bottom layer of water will allow every tile of water above it to teleport into the chamber (5-10 ticks at most), but cannot itself leave the cistern because it lacks the pressure to do so***
3. Toggle the airlock again to refill the cistern while you wait for the goblinite to stop twitching.
4. Open the drain. All the water on the drowning floor will teleport away, through the underfloor and past the pump bypass, again in 5-10 ticks. No puddles to worry about, even!
5. If any amphibians swam to the level below, pump it out and send in the military to mop them up. A grate over the pump's inlet keeps them from escaping through the drain. Otherwise, you can leave the water there for next time.
5. Open the service door(s) and collect the goodies (I drew a door only for clarity, they should actually be drawbridges to avoid problems with building destroyers).
** Stairs turn out to be really nice. Flowing water doesn't wash items past them, and they're absurdly easy to dig out compared with building bridges or grates. Plus, they're immune to everything, even building destroyers and dragonfire, and they can't grow trees. They're effectively impassable to pedestrians. Fliers can traverse them while dry, but would have nowhere to go with the drain closed. Amphibians can traverse them when flooded, but they're still trapped by the pump's grate, and you can always pump out the bottom level to get at them.
*** this chamber cannot flood even if you forgot to close the ceiling. Good thing, because I've forgotten before...
I haven't had a chance to try it on real goblinite yet, but "dry runs" using random items I dumped in there are very promising.