Nice sprites; I liked the limited color palette, and the shark's subtle swimming motion.
I'm not sure why the oxygen meter never increased, even when I was swimming (the best I could do was to keep it at a level).
Also, at some point I kept swimming, but could find no prey. I suspect I had gone out of the bounds of the scene (or was bumping up against them), but I had no clear feedback about this. The bounds of the scene should be indicated somehow.