The only manga I've read on the ocean theme is Children of the Sea, which was slow, but had some beautiful scenes and art. I haven't seen that anime, A Lull in the Sea, though I just went an looked at the trailer. The art and animation is amazing on that! Except for the too cute faces.
With Hydro Man's powers, I can see Namor doing that now. In some ways, the original black suit was doing something similar. I do find the idea of using his new powers could eventually cause him pain, as a limiting factor and a symbol of heroism. But I don't want it to devolve into that mess that Jason Aaron did with the Vomiting Dr. Strange. Just NO.
Aaaaah! The oxygen imbalance. I hate that theory. I'd much rather have Namor be responsible for his actions.