Really great representation of the theme! It's super simple with upgrades that encourage emergent gameplay and trying different combinations. I especially loved making a super long fish.
There were a few things that felt frustrating to me. First, it kinda sucked that without any Lung upgrades you lose about half your lungs before even leaving the fish tank. Anytime there was an obstacle felt really frustrating since it was random whether they were there or not. It got to the point where I would basically just stop playing if I hit an obstacle that I couldn't overcome quickly since next time I'd likely not have to deal with the obstacle. I would've preferred them to always be there or always not be there. Lastly, of course I would've loved a win screen.
I also wished there was some music! The art was really charming, especially with the different backgrounds. The way the fish grew and changed based on your choices was super cool. This is a really cool game. Fantastic work :)