I really liked the idea of switching between land and water to balance your oxygen and temperature. I got to about 1200 before I resurfaced. I think an indicator sound when your oxygen is too low or temp too high would be nice. As you go deeper, the difficulty does increase because the number of blocks that you have to break are higher but maybe you can also increase the difficulty by "locking" the wall in between at certain points so you have to time your switches better. In the tutorial, I would suggest that you just show the text on top without having to click to continue so the game isn't interrupted and you don't accidentally hide the text. All in all, a really good game with scope for a bigger one. Good job!