Could use some improvement.
Imma give this a 5, as you have a good base for a game. This could use a bit of an improvement though. It is relatively slow paced and a bit boring. One other thing is that i think it could use is a quick how to or something along those lines. It doesn't have to be much, even a "Use the arrow keys to swim forward" before you go into the gameplay type of thing. For this to be a good game, you need to add more to it though. Maybe cut it in half and use it as an intro for a later game or something. Standing alone though, there is not nearly enough to it. Just my opinion.