That little merman is probably the ocean scientist that we were told went missing.
As for what will happen next, I'm guessing some BS then he will be back on land like nothing ever happened,
although he would have matured emotionally and settles down with the girl thats acutely pining for him.
But seriously if its a happy go lucky bs ending, then I would have proffered it end after the first volume; where we has about to get eaten and pushed her away. That dark bitter sweet romance is kinda what I came here for in the first place.