This game is simple but fun! I got a score somewhere over 1000 before I died, so it's definitely possible to score but not too easy. The movement for the dolphin feels very nice, and at first it was a little jarring how he moves slower to the left than the right, but I got used to it and I understand why you made that decision. The fish are fun to collect and give good feedback. One suggestion I would have is to make the higher scoring fish move faster so it is a greater challenge and more rewarding to collect because on the surface it felt like it was just as easy to collect as the regular fish. I think my only suggestion for your game is about the visuals, as the art is very pixelated and doesn't look to be made that way. I think either making the sprites higher resolution or finding pixel art versions of the graphics would greatly improve the visuals. I also noticed the background has the same issue, but it also doesn't seamlessly loop. I think you should look into @jjdeglow 's suggestion on fixing that. Overall it is a very fun game to play!
Also please consider adding the game to be playable in browser, rather than having to download it, extract it, and play it with a live server in VS.