Okay, after asking a modly:

Ryujin is a Japanese dragon god of the sea so it could play into Percy's normal abilities but could also eventually let him turn into a dragon. With time, he could even grow more powerful than in canon by creating and using tide jewels that allow him to control the tides with little effort.

For quirk: perhaps when he gets angry or upset, he has less control over his water-based abilities. Nearby water starts sloshin as if it were in the middle of a fierce storm.