The current physics you're using feels wrong. Right now, the sub descends at a constant velocity and pressing space instantly makes it rise at a current velocity. While the speed at which is rises and falls is fine (it's your game, after all), I'd add some acceleration to it to make it feel more like a real sub. When a sub begins to climb while descending, it slows to a stop, then begins to climb.
Suggestion Implement some acceleration into your movement to naturally change from ascending to descending.
Bonus Points Change the sub image's
angle slightly as it rises and drops for even more gamefeel!
I'm getting a full 60FPS. This is great.
This is a touchy subject — literally! If I barely touch anything, I lose. This seems to be based off the hitbox of the sub (which is fine), but I'd shrink that hitbox quite a bit. I felt like I should have been able to scrape past some of the seaweed after nabbing a tricky fish, but I'd crash, even though the seaweed hit above and behind my tail.
Suggestion Make the hitbox fit entirely within the pixels of the graphic and shrink it a bit.
You're being chased by a shark! The shark will slowly catch up with you unless you do something about it. Depth charge barrels now appear in addition to fish. Touching a depth charge will trigger it, knocking him back and slowing him down momentarily. While being chased, all points and fish are tripled! Survive as long as you can without getting bitten or hitting seaweed!