Nah. Maybe if it were a ridiculously cheap small wonder (like 60 or 70 shields) that put a coastal fortress in each of your cities, it'd be worth it? (and coastal fortresses cannot be built otherwise)
Possibly. It'd be slightly better bang for the buck, so I guess I'd be slightly more likely to build it. Still mostly useless though.
but they sound so badass! COASTAL!! FORRRTTRESS!!! Mhuuuhhahahahahahaa!!!
Anyway.
I've made the thing into a Dromon building (1 every 5 turns, just like the Temples of Zeus and the Knights Templar) Small Wonder that can be obtained by researching Invention.
Industrialization makes it obsolete.
Not a bad idea, nor is the cannon one.
Now I'm tempted to make it produce Privateers, because there aren't usually enough of those in the game. More realistic might be an immobile, range-1 missile unit, but I don't think you could limit it to just the sea.