There's iSail, which only has 37 downloads so it would have to be tested before you add it to the main server. There are also a few normal mods.
http://www.minecraftforum.net/topic/1857899-162-152forge-archimedes-ships-v13-infinite-airships/Doesn't have cannons or anything, but ships are really all I want.
Are you considering adding things like NPCs such as Shop owners? I mean the Market is basically useless right now. There are other neat mods that I would like to have on the server. I don't like the idea of having any more dimensions or a ton of new items, but there are a few things that I think would enhance the experience. Just let me know if you're even open to the idea and I'll test them out tomorrow night.