Very smooth, and runs without any problems at all!
My favorite bit of the game is the gorgeous lighting for sure. The flying, green glowing particles look really cool on the screen, as does the light function. I will say I was confused as to how the light function worked at first, for some reason I initially thought it was pinging shells I hadn't uncovered yet... I'm pretty sure this is a me-issue though, lol. I also loved the glow-y look of the shells, pretty stuff!
The player's movements feel so smooth and well-built, and I enjoyed running around a lot because of how smooth it all felt! The jump is my favorite, it's so so so smooth and slightly slowed, which helped make it feel properly underwater!
To be honest I quite liked how dark it got, even if disorienting, because it feels like the point of this game is to be a little disorienting. Like you have to go out of your way to dispel the darkness with the light feature. Otherwise, it'd be far too easy to find the shells.
Overall, I have to say that the controls are smooth, and the lighting is beautiful!!! Really enjoy the look you guys went for.
Awesome platformer :)