Pressing space while underwater should definitely be a secondary control for 'up'.
Also landing that 'bump your head on the ceiling' jump in the upgraded gills level while dealing with 2 unkillable monsters is pretty annoying.
The monsters are fine, it's just that style of jump I feel isnt really testing my platforming and is immersion breaking.
I really enjoyed the Line of Sight mechanic.
This ran fine on my machine.