Because screw the ocean's ecology. Take that, stupid ocean.
@SalvySaroj It's weird, right? This manga is almost like anti-fanservice. Even in the (mandatory) beach chapter--with tentacoo wape, no less--it didn't get any more fanservice-y than rather modest swimwear. Rather than the stereotypical 'afraid of a woman's touch' herbivore MC, we get one with very valid reasons for keeping it in his pants.