Or even pack the ship full of cows and peasants, and make a floating farm
I somehow have trouble picturing that :-P
Couldn't you make that somewhat less illogical by just implementing fishing for peasants on boats?
Also, you didn't answer my question specifically with regards to the performance of siege units placed boats. How will a catapult or trebuchet operate differently on a boat than they otherwise would have on land? Can they fire while the boat is in motion? Will they have the same range/rate of fire? Will there be some kind of ammunition limitation for these weapons?