The game looks really beautiful. I feel like I'm playing a long-lost GBA ROM. It reminds me of the Flashback Legends demo if you know that?
The air physics could do with some improvement. Jumping and falling feel very "floaty". You move up and down at a constant rate with no "arc" of slowing down in the middle of the jump as your direction reverses from up to down. Falling feels too slow.
The jump's spin animation doesn't feel very smooth, like you can tell it's separate frames of animation. Perhaps it needs more frames? I am not sure.
The default controls feel uncomfortable to me, I found [Z jump] [X shoot] [Shift run] to be more comfortable on a keyboard. Gamepad support would help.
The enemy movement is not great. If the flying bugs hit you, they jitter back and forth very fast. The big jumping enemies feel pointless early on with such a weak gun, their art is also inconsistent with the rest of the look, as those monsters are hugely pixelated compared to the rest of the environment.
You're obviously well-versed in Metroidvanias so the idea of being able to just ball-jump and swim straight away feel like a shortcut. You know how long it took to get those that in Super Metroid :P However perhaps I am comparing your game too strongly to Super Metroid.
Please don't take this as too negative. Overall this is pretty amazing. You've got 2/3rds of the way to making your own game which is easily better than NES Metroid or GB Metroid 2, and is approaching the quality of professional games like Super Metroid or Zero Mission. That's a great achievement! It just needs a little more polish to be great.
I hope that is helpful feedback. Please keep making games! Imagine how good you will be in like 5-10 years with more experience.