I'm a bit disappointed here, the mechanics seem quite barebones.
Naval tactics could be a bit more diverse. What I can think of is that Ramming should increase damage dealt and received and boarding should make it possible to capture ships. One tactic could be to use archers to fire from a far, which would decrease damage dealt and received.
With pirates there could be a lot better and interesting mechanics. For example: Make it possible for ships to become loyal to admirals like armies can be to generals. Disloyal admiral could turn into a pirate, taking ships loyal to him to his pirate fleet. The fleet needs a place to harbor, so nearby (or not so nearby) port owning nations are asked one by one, until one agrees , if they would let the fleet use one of their harbors as a base of operations. The nation hosting the pirates would get a part of the plunder the pirates hoard. It could also generate events in the province/city with the harbor (mostly negative, put positive aswell. Maybe having an army or navy at the harbor would keep the pirates in control and deter negative events). The nations which get plundered would get a cacus belli towards the host nation. Something like this would be much more interesting and tie pirates to other mechanics of the game, instead of making it just a random chance.
There could also be a lot more cacus bellis and wargoals.