Boundless

Haruka, a wandering water god, just wants to find a stretch of water to settle in and call home - preferably one big enough that the humans can't erase. He finds an ideal site in the gentle harbor of Iwatobi, recently vacated by its original god some human years ago, but a big stretch of ocean is more than his power can handle for the moment, and an aggressive god named Rin is already trying to beat him to it. He'll need followers to gain that kind of power, and to get followers, he needs a Priest. He finds an ideal one in Tachibana Makoto, a gentle human and captain of the swim club that uses the pool Haru decides to bunk in until he has enough power to move to the ocean. Now, if he could only convince the green-eyed human that he's not a ghost trying to drown him, things would go better.

AKA the AU where Haru's a god and is trying to beat that stupid red-haired shark spirit to settling into the ocean and regularly scares the bejeezus out of Makoto by popping out of his school pool like Sadako.