I really enjoy the marine pollution theme of this game. It's a very serious issue that deserves more attention. The gameplay is good, but I did encounter some issues that I think you should address.
When the player enters the water, the movement feels glitchy and it seems like the player is stuck. Also, jumping over obstacles on land is a bit difficult when using the spacebar, but it's okay when using the up arrow.
I carefully read the tutorial, and I know that you specified to press Enter when reaching the end of the level, but it would be helpful to remind the player with an icon or a text prompt at the end of every level to make it easier and more intuitive to move to the next level.
Additionally, I found that the tutorial prompts appear too slowly. It would be better to freeze the game to give the player time to read the tutorial and understand what to do. I tend to play quickly, and I had already almost finished the tutorial level before the text prompts appeared. Also, it would be helpful to add a warning about the hostile anglerfish, even though they don't do much damage.
Lastly, I had difficulty distinguishing the healthy fish from the sick ones, especially if they were a different color than the standard ones, and I promise I'm not color blind.
In conclusion, I really like this game, especially the ocean-cleaning mechanics, but it could use some tweaks. Well done!