Aight m8, there's a sequence of events, maybe you missed something.
But also, make sure you're in a recent version of the game.
First, you meet one of the fish girls in the main story, eldritch branch.
She teaches you a power to breathe underwater.
Now you should explore in the mountains (map where Gramps lives)
You go W->W->S and find the second fish.
With the power you can have a scene with her.
Now, you have the Marina. Which for the record, have to talk to Madison in her mansion to be able to buy it (after getting her to 4 hearts)
You buy the Marina and also get the tunnel upgrade.
With all this done, you should have a new option in the Marina.