I immediately think of the floating villages of Waterworld (great movie, don't care what anyone says). I'm not sure why they would need to catch all of the fish so quickly. Wouldn't they be better off to catch them slowly, enough for the town to eat off of? Or are we assuming the floating city is so big as to need huge catches in a short period of time to survive? If that's the case, you'd have to come up with a convincing reason for anyone to want to be on the ship, let alone how it travels so quickly. I personally prefer the idea of a small village ship, one that might even have their own aquatic agriculture: growing kelp, herding schools of fish, etc. That's my two pence, anyway.