Don't they always do something?
Anyway, this issue is a tricky one, as I come to think of it... imagine you make yourself 5 boats which only you can drive, then you use them to block something, the swimming way to Hotaru, or a kingdom's shipyard (if they were used). Another example: you own a boat and use it like a house, store your stuff in the hold, then someone comes by and presents your home with 30 cannonballs - everything's gone, what now? I hope you see what I mean...