It Begins with the Mermaid Revolution

For reasons the author can't be bothered to explain, a young man transmigrates to Tokyo in a parallel universe. He gets the standard orphan setup, complete with a house and savings, plus a childhood friend next door: Sora. One day, while out with Sora's family, they are involved in a bizarre car accident. Time suddenly stops, and he appears on the sea as a member of the merfolk, in her identity.

Fanfiction. The first volume is One Piece. The main world is a multi-series crossover leaning toward slice-of-life anime, with another world primarily based on Chinese animation mixed in,