I interpret the last scene as a rape/suicide. The mermaid has a knife in her hand and a hole in her chest... The male seems to be either assaulting her or trying to rescue her. It's open for interpretation.
They are both men, you can tell. The mermaid has a muscular masculine frame. Obviously male. And the 'girl' is the man who was helpinf the mermaid by pouring water into the tub, duh.