when a stranger swims past you and unexpectedly and unknowingly brushes your genitalia.
I was at the town pool on a hot summers day when my junk got water rubbed by an old man's fleshly calf as he swam past me
Masturbating while fully submerged under water. This can be done with one's dominant hand or the hose end of a snorkel when appropriate. Water rubs date back to the times of the ancient Greeks and can be seen practiced today among all of the world's oceanic cultures.
Mom's voice calling from just beyond the wave pool forced me to cut my water rub short.