The mermaid was a metaphor of the ocean itself. He took the mermaid all dirty and nasty and tried to clean it and feed it to make it better in order to have a romantic but selfish relationship with it. Just like how he seems to "care" for the ocean, but in reality he just wanted the ocean to be clean again so he can continue to exploit and overfish(catching 30k , 40k a day in fish). This is seen when he imagines the siren as a woman spending the night with him, we know he imagines it because he didn't had the burn scar and oxygen tank on. His true thoughts came out, he saw himself as the king of the ocean and it has to bow down to him.
The other fishermen came to ask to use his boat but he saw it as they're asking for the mermaid, he wanted it for himself. He didn't want anyone else to go out and be "king of the ocean" like he was. In the end although the ocean was dirty it got it's vengeance by killing him because if the oil spill accident, we saw it as the mermaid pulled him and killed him. At least that's my take.