It's unfortunately really hard to see and really hard to catch anything. It would maybe be better if the fish were color coded, so it's easier to see what is prey (other than them just swimming away). Most of the time I just die immediately because I slam into a large fish as it lurches out from the dark. Even when I thought I was doing good, I just died to a fish that I couldn't see was chasing me. Also, it's hard to tell how much you're moving because the background is unchanging (maybe some repeating dark background structures would help, akin to Hero in the Ocean). There are some good things about this game. I like the mouse controls, they're pretty straightforward and responsive. The visual style is pretty good too, albeit a little plain. Overall, this is a good first flash game with a lot of potential, but at the end of the day it's just overly difficult as a result of the vignette and the behavior of the fish.
I do look forward to any revisions if you decide to continue developing this.