Thanks for the feedback! I'm glad you liked the game. I'm also laughing to myself because I wasn't sure if anyone would figure out this method of cheesing the game :)
I debated with myself for a while if I should offer a way out of the low oxygen situation or if I should just leave it up to chance. In the end, I added some padding between "emergency oxygen" sections and normal sections to increase the chance that the player would reach the oxygen at the end of the grace period.
I think what I actually need to do with the colliders is fix the player's shape. I stuck with a capsule collider because I had planned to animate the diver and wanted to keep the colliders simple. But it's shoulders and the feet have way more space around them than they should. I'll leave it as is for the review period and fix it afterwards.