Some info I got from this chapter:
During the underwater combat he thought:
It was really frickin' hard to swing my weapon when it wasn't part of using a skill.
So swinging his weapon underwater was hard, but when using skills there was no dificulty.
This means skills can force the movements even while submerged in water. This isn't such a big revelation, but it can become useful when trying cheese a system like how speedrunners can use forced animations to cheese a game.