Like motor-boating... but with an asshole.
Bro, I was totally steamboating that girl all last night *high five*
having sex with a girl doggy-style in the pool, in deep enough water that she has to keep paddling to stay afloat.
"I was in the pool with Catherine, and we ended up steamboating! The huffing and puffing when she ran out of breath only made it better!"