Obie Is Man Enough by Schuyler Bailar

A coming-of-age story about transgender tween Obie, who didn't think being himself would cause such a splash. For fans of Alex Gino's George and Lisa Bunker's Felix Yz.

Obie knew his transition would have ripple effects. He has to leave his swim coach, his pool, and his best friends. But it’s time for Obie to find where he truly belongs.
 
As Obie dives into a new team, though, things are strange. Obie always felt at home in the water, but now he can’t get his old coach out of his head. Even worse are the bullies that wait in the locker room and on the pool deck. Luckily, Obie has family behind him. And maybe some new friends too, including Charlie, his first crush. Obie is ready to prove he can be one of the fastest boys in the water—to his coach, his critics, and his biggest competition: himself.