Ningyo no Ouji-sama

From 200 wpm:

Once upon a time, there was a mermaid: "I want to become human so I can fall in love" she said. When her wish was granted, the lower half of her body turned to foam and she became a human... MALE ♂!?

This is the love story of a 30-year-old virgin man and a beautiful (male?) mermaid!

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From LuffyNoTomo:

A 30 year old man named Koganei, Manato is down on his luck. He can't seem to get a job, has little money, is still a virgin and is giving up on his dream. Sitting alone on a swan boat in a pond next to a shrine, he holds a lottery ticket thinking this is the only way his dream may come true. But the wind blows it away and he falls into the pond, thereupon seeing a beautiful mermaid!

Manato ends up going back to the pond and after the mermaid decided not to kill him since he gave her food, she tells him about her dream to become human and how she wants to find love (she collects coins and throws them to a shrine in order for her wish to come true). Manato decides he doesn't want to get involved anymore because he has his own worries, but after another bad day he goes back to the pond. She finds him again and after listening to his complaints, she decides to give him all her saved up coins in order for his wish to come true. She suddenly turns into bubbles and disappears.

Confused and upset, Manato hears a strange voice and a god from the shrine comes to say that she will take the coins and grant him a wish. He decides to use his wish to bring back the mermaid as a human. His wish comes true and he sees the mermaid back as a human, but she's a guy! What will happen in this crazy love comedy!?