There are multiple ways to prevent the player from falling into the ocean. You can put a box collider 2D right at the shore of the island.
Another thing I noticed is that the submarine is ignoring colliders entirely and instead is just forced out of sprites, which makes me believe you are just changing the submarine's position depending on input. You can add a Rigidbody 2D and directly change its velocity to get similar results, but with proper collision detection.
I also notice that when you use KB to move, there is some gradual acceleration but when I use a controller, the acceleration is instant. Mid-air movement with the character is a bit too floaty. I believe both of these 2 things don't matter as much, but I wanted to point that out.
Overall, it is not a horrible entry. Falling for eternities while the player character distorts out was a treat to watch
EDIT: I accidentally posted this 3 times, sorry about that
Also for those who don't know how to play this game, there's a good amount of info on the Game page itself.