All of the following is my subjective opinion on your game. Thank you for creating it.
These are just my suggestions, take them as you wish.
To be brief, the idea is interesting, but the realization is not the best.
To be complete, here is a list of my suggestions.
1. The surfacing and diving take too long(20-22 seconds each), and there is no speed enhancement in the store, such as scuba or flippers.
2. Weak progression: for money you can only increase the amount of oxygen. You can add to the store, for example, things from the first point. In addition, I noticed that the value of treasures (how much money they give) during the game does not change.
2.1) All items are on a flat surface. If, for example, the map was a “tapering mountain” (like in the screenshot), where the value of the item depends on the depth, it would be a little more interesting.
3) Gameplay at the moment is a bit monotonous in the sense that nothing in it does not change, except the duration of stay in the water. You could add things that will hinder the player, such as sharks that will chase the player and can even kill.
4) Minor bug: the boat is not visible underwater.