It's fair to consider that she should be returned to the sea, but it should be noted that the sea (more accurately, a typhoon) spat out a mermaid child, wounded and unconscious, onto a beach along with piles of trash. This, and the fact that she broke into tears upon finding herself alone back in the sea, suggests that she'd rather not go back in there just yet. For all we know, the trash that washed up with her might have already ruined her habitat, and the typhoon may have done more emotional harm than just a scratch on her arm. It may have even claimed the lives of her friends.
He can feasibly raise her when he sorts his life out, with occasional covert trips to the sea at night to help her get used to it again, as well as teaching her language so she can decide if she wants to go back or stay. There are hurdles to overcome along the way, but this is what this work seems to be about in the end.