Insanely hard! You are a mad man my friend :) But the game is rewarding if you manage to avoid some traps. The character is moving very fast, which make it super hard to avoid things sometimes, but I assume its intentional. Game is super challeging but those challenges are kinda addictive. I liked it! Also traps are well design, because apart from their hardness seems fair and beatable if you learn how to do it.
I noticed a bug if you swim to the left from the begining of the game, you are falling off the map.
First I also drowned with no chance to continue, but I read your comment about the healthkit spawn, and continue. I stuck in the room, where it was unable to jump because i got celling near my head (I think I has to be made with some other power up). Maybe later I will try to continue, but for now i need to rest to calm my nerves :D