Considering the hi-score table was hacked before this game could even get into the portal, I think you should seriously consider adding some kind of encryption and validation to the hi-score system. The gameplay was okay, with every object on the screen providing its own risk-reward schedule, and a diminishing returns curve to the scoring system no less fair than your typical MMO's levelling mechanism.
If you want to move this from being a decent game to a great game, consider working in some sort of powerups, especially if it means you can specialize your fish's abilities.
I would like to add that I think it was the music that kept me playing for as long as I did. Even when nothing new was happening after a while. Somehow I think the suspenseful music kept me psychologically convinced that something cool might happen any second now, so I kept playing longer than I probably would have otherwise. I should point out that if the controls were unresponsive or the gameplay was frustrating or unfair, the music wouldn't have saved it. But the controls were decent, and the gameplay left enough wiggle room for me to plan my moves the way I wanted to.
The character designs were quirky, but since nobody talked or had any apparent goals, they didn't really interest me at all. Maybe next time try to add a narrative of some sort. Just a thought.
Overall, good job!