I feel like sea provinces should get features like land one do (I think I saw some rare ones but usually sea provinces are completely empty). E.g. land provinces often get: Gold Vein, Pastures, etc. which makes you prioritize them.
Also I kinda feel like sea improvements are kinda disconnected from the rest of the game in general.
Like when you build land provinces improvements they boost some buildings making it easier to build them.
But sea province only boost one specific port building. You don't get any special province improvement for sea provinces as well...