Water polo suit. Gotta have one to play...but likely more than one. Wearing the same suit every day is gross.

Water polo cap. Punching, scratching, and hair-pulling? Yeah, no thanks.

Water polo ball. For obvious reasons.

Cages. The goal of the game is to score goals. A physical goal is an important part of this process. Water polo goals are called cages. Phewf, that got complicated.

A pool. We tried playing water polo on land once. We don't recommend it. 

Bladder control. Don't be that person who shamelessly pees in the pool. Just don't.