Underwater Jacking 

I believe a better term for wanking in the tub would be "Bathturbation"

Last night Dan had a bath and performed Underwater Jacking aka Bathturbation

Underwater Jacking by lizard_lightning March 21, 2015

Underwater Jacking 

To jack off under water in the tub where no one in the house can hear it. Plus it feels really good

Son - Mom I need to take a bath.
Mom - But hun, you just took one?
Son - I need to go underwater Jacking

Underwater Jacking by ItsMaybeEthernet March 9, 2015