Game looks pretty good . I'd appreciate it a lot if you could check my game too

There is no strategy. You just figure out the optimum time of pressing the button, and then you just press in that rhythm.

Interesting game concept. Some things I'd suggest

1. Create a Pause Menu that allows the player to go back to the main menu. Not sure why but sometimes when I tried to click on the button for level 2, it pressed tutorial.
2. UI Buttons should change color more noticeably when highlighted.
   
3. Add some obstacles in your levels and allow your character to move upward and downward instead of just simply going straight. You can use the arrow keys for this.
4. Link your breath action to a keyboard button like spacebar
5. Something fun I thought of was to have some sort of music that plays based on the players distance from the shark (if you're further than certain distance then the volume is just 0%, but if he's right on your tail play volume at 100%)

Possible Bugs:

• When the player slows down from lack of oxygen he eventually starts going backwards.

The idea is great, needs to be evolved. I don't get why the player shouldn't just click a breath button when the progress bar is on 2/3. So, I guess you need to add something, that motivates the player to get a risk to be eaten by a shark. For example, add stamina and make stamina-bonuses visible only when player oxygen is under 50% or something like this. And yep, we need more variations of levels! :)

the concept is quite interesting, but i think it needs some balancing. if you manage to hit the right spot, the shark has no chance. maybe the shark could get movement bursts or something similar? somehow i managed to swim backwards during the tutorial :D

Love the concept!