Just a fun little concept I found a way to make. Always thought it was impossible. Turns out not. These player controlled fish can swim through one block high gaps!
Actually, my plan is to make a co-op puzzle map where one player is a human and another is a fish as seen in the video, and they have to trigger different things and do stuff together to solve the chambers. Only problem is, it takes so much time to make each chamber and the mechanics and the water pipe :/