This is a lovely prototype! I think it's totally underrated, there's huge potential in this game - I have some recommendations :D
1) The controls were a little hard to manage (but we already talked about that, certainly didn't affect my rating), so that would need to be polished imo for a seamless experience.
2) The gameplay felt lengthy, even though I could see how much trash I had collected. If you don't mind, I'm going to link you to another game here as an example of voice over narration you can do every time you hit a certain threshold, they did it very well and it's a similar theme: Echoes Vol. 1 by Plume Pond (itch.io) - I think if you included something like this it would be very motivating to play.
3) With that kind of narrative added, you could easily tell an emotional climate change story about pollution, and uncover a mystery halfway through. I was lacking some sort of reward system even though cleaning the ocean felt very therapeutic. I also like that your game keeps it chill and casual, and that I'm not fearing for my life every-time I run into a rock. That's a great part about this game I would definitely keep.
4) The controls being displayed at all times is super helpful in these types of games. Great tutorial in that sense.
5) I think you'd benefit from adding a sort of echo or sonar where you can detect from above where there is trash so you can dive there. I would also include less trash, it seems a bit endless at times.
Graphics and design amazing!