Nice job, but it's quite hard! I only got the level 1-5. I like the art, and the game feels quite solid, but there's some tweaks I'd personally make. I understand that it's meant to be a precision platformer, but I feel like certain parts are a bit too precise. This is only my opinion, maybe you enjoy this type of precise gameplay. One area is the lack of coyote time. In lots of platformers, you have a few frames to press the jump button after you walk off a platform, which means that you don't have to be quite so perfect with the timing of a jump, you get a couple frames of wiggle room. Even the game celesete, which is a fairly difficult game has this feature:
One thing that I really think should change is either the size of the fish's collisionshape, or the player's collisionshape, that's the part I got stuck on. The collisions kept happening when they felt like they shouldn't, and trying to precisely put the player in between the fishes on either side on level 1-5 to not get hit is really difficult because of the lack of analogue controls. Usually hitboxes are a little bit smaller so you can get away with slightly scraping the edge of an enemy, but in this it felt like the hitbox was as big as it could be.
I feel like these changes would not take much away from the challenge, but would add some flexibility and make it feel a bit better because of the slightly extra room for error.
Overall, great job on finishing the game! The animations are nice, the player feels good to control, and from what I've seen the levels are fairly fun, but the things I mentioned made it very challenging, so I wasn't really able to get a full opinion on all of the game.
Also, maybe adding variable jump height could be a good choice, allowing you to make short jumps, instead of doing a full jump every time you press the button. This would make hopping short gaps feel a bit nicer in my opinion, but I suppose it's just a gameplay choice, not something that's necessary. And like some other people said, controller support would be great.
Update: I played a bit more and got to level 1-6, but the aiming was just too difficult. I think having a cursor on screen that the trunk points to would be a bit nicer, at the moment, the aiming is a bit unintuitive.