Aye, it would be an awesome P101-Only feature. It would also require much more ship bosses to farm; the current ones won't be enough for an active Nautical-PVP System.
A ship pvp would be a nice addition in pirates. A ship arena would be nice to place near Floatsome town, but then again free to play players would complain. Where would be a good place in the free zone for a ship arena ? A ship pvp would be something unique to pirate101 players, if it's added.