Wave mouse left + right to flick your tail and swim forward (or flop forward if you're beached... and if you're beached, you need to flick your tail extra hard to get back to water!)
WASD to guide the direction you swim (or what direction you flop forward when out of water).
R to reset if you get stuck / lost.
K to kill the fish you're playing as (no idea why you'd want to do this, but I used it for testing purposes so I figured to leave it for now).
F to refill your energy completely.
If you go into 3rd person view, you're being attacked and are quickly losing energy, so wave your mouse left + right fast to escape!
Tiny glowing fish are a lure. If you get close enough, your fishy instincts will automatically pull you towards it, unless you use your survival instinct (your IRL brain :P) to swim away to a safe distance. But they're your only source of food, so you'd better go for them anyway ;)