Good game! I was really excited to unlock the last door when I got the last key! It made my Metroidvania side happy, but I didn't feel a ton by way of mood-shift when I fell in the water. Having tried a similar section in my Toad Mountain game I can appreciate that it's a difficult effect to achieve, and on a psychological level I appreciated that the fish was in water, but I think harsh music when next to fire, perhaps starting out in water, or even having a meter that started depleting when the fish was out of water that killed it when it reached zero could have helped.
This all being said I thoroughly enjoyed beating the game in 1632s, I think falling into water rather than just land is a welcome extra mechanic to add to the Getting Over It formula, and the keys, though perhaps distracting from the intended mood-switch, gave me a lot of joy. 4/5!