I thought this game was quite charming. The graphics, while simple, definitely conveyed the feeling of being under water and the swimming mechanics were smooth.
A couple small observations… I think if the cliff faces had a little more roughness/texture to them, they would have helped the environment pop just that little extra.
The other small thing was, while the swimming mechanics were good, there were a number of times I wisher for a “swim up” and “swim down” key… Going up and down based of facing is good and should still be there, but having a little vertical strafing as well as horizontal would have made me feel even more comfortable with the navigation.
Over all, great work, in my opinion!!