This was a really cute game, love the lil mushroom guy lol. Getting legs too was very funny. I also appreciate starting with absolutely nothing (I tried to do a similar thing in my game too). I think it would have helped ever so slightly to show the player what button to press as soon as they get the powerup, but it was designed in a way (at least with the legs and arms) that I couldn't really leave until I had opened the inventory and figured it out, so I can't really complain!
I especially loved your enemy designs, not just artistically but I thought they all had very unique and engaging movesets! There was a staggering amount of variety too! I also appreciate stuff like the spore mushroom platforms if those count as enemies, simple but good ideas.
I would recommend putting a timer on the water, or depleting health over time. It's clear that you probably will die, however it wasn't eeentirely clear that I was gonna die despite flashing low oxygen. Or rather, the danger feels less tangible. It's a little troubling on stuff like the part where you need to wait in a pool for the saw to pass overhead
The scope of this game was huge! Big map, 3 bosses?? I played for like, an hour and I didn't even get to the other 2 bosses. Made some ways in to the beach before I got exhausted. There is a looot of enemies and sometimes there aren't many breaks between them. At some point I just give up and go "I'm just gonna damage boost through this because it's going to take way too many hits, TWO totems right next to each other after this long run, no thanks"
Besides that, I think it could have benefited a looot from a map, it's very easy to get lost and retread old ground without having idea where to look next or where you've already been. Congrats on making something this big and varied, I'm interested to get to the other bosses maybe when I have time later, the mosquito boss was well put together and challenging!