Hi,
It's your game but I feel like scifi games are on the rise the last couple years, look at Star Citizen, FTL, Elite Dangerous, Starbound, Stellaris, Endless Space, X-Rebirth, EVERSPACE, ect ect
I'm not sure making it a submarine one will net you a lot of users/customers since to be honest I would never go out looking for a water based game, I found this game yesterday by looking for a multiplayer space game since my Star Citizen organization (Clan) is waiting for the next patch and I've stumbled upon this game.
So my advice would be to keep it Sci-Fi orientated, since you are probably planning on keeping the current engine you could very well add the ability to allow spaceship to land and submerge into ocean planets (requiring specific parts or upgrades) or have a smaller craft go to the planet and submerge.
Underwater planets could contain dangerous life forms but allow for extracting rare minerals from the sea floor or exploring underwater constructs that hold tech.
This would allow you to give a creative underwater twist while still having the ability to jump between systems using space ships. You don't have to put a planet in every system make them feel special.
Just my suggestion after reading your post.
~DGM