Sushin,
Had another try and it all worked fine, so I added some feedback below :)
1. I found it fairly easy to work out the controls, although I think it could help to add an info box or short tutorial to show the user how to play!
2. I thought the point of the game was to clear the mines out of the underwater habitat?
3. It was probably a little bit too hard for a first level.
4. I did find a couple of bugs; A. at one point my air meter glitched and the bar grew to twice its normal size. This worked out well for me as I could spend twice as long underwater. But I'm pretty sure it wasn't meant to happen. B. when I died I was still able to carry on playing even though the screen said "Game Over".
5. I stopped playing after a couple of tries, it was a bit difficult to reach the end with such a limited air supply.
6. A tutorial to help get players started. A bigger air supply in the early levels (this will give first time players an opportunity to get in to the game, and get hooked on it!). And as JAC mentioned perhaps improved surface swimming animations.
Hope that helps you, and good luck with the project ;)