This game held some potential to be interesting but let me down in a few areas.
I'll milk the good before I go on to say where this game failed to draw me in.
First of all, the difficulty is reminiscent of Flappy Bird. I enjoy that the velocity is not overwritten by a fixed amount upon clicking the dive button but instead "added" to the existing force that is being applied to the character object. This creates tension for the player as they are being drawn to the top and must fight against the velocity to bring them to a stable and consistent alternation between two values; very similar to Flappy Bird.
It was also accurate to have the player floating to the top of the screen rather than the bottom, as oxygen rises under water, and so a human would also.
Unfortunately, some of the mechanics are the only good things that I have to say about it. The player should not die upon reaching the top without cause; and the same with the bottom.
Obviously this would detract from your game design, so let's say we can overlook that— it is still poorly implemented as there's no direct line for the player to know /where/ their game will end. It kind of just... happens if you get kinda close. There should be a distinct way of telling the player.
The music loop is bad. I understand there's meant to be an echo effect, and props to you for creating that to fit the game as best you can, but it is irritating. Thank you for the "mute" feature. In the future, I would spend more time creating a more easy going tune with more variation.
The player should know how to play! Tell us to click the "DIVE" button. We don't know, it just looks like text that could be equivalent to "GAME START!" — That was my first impression. A feature to use the spacebar instead of clicking would be nice.
Bringing the scene to life would also improve the quality. This world felt very stagnant; the sharks didn't move, the player didn't swim, there were no fish and no life. Just the same scrolling background with the same...
Sharks and collectables! Every single game the sharks and collectables were in the same spot at the same point in time. Create new objects at a random pace, at a random Y value on the screen, such that it gets harder over time if you're worried about difficulty. Give the player some replay value. It's ridiculously easy with this genre.
With all that in mind, the difficulty is pleasant (and I challenge anyone else to beat my pathetic high score of 18) and hopefully my advice in regards to your room for improvement will help you on your next project.