sink the boat

The act of taking a crap and using your urine to cut the feces in half.

'Dude, did you here about this fun game you can do in the bathroom called 'sink the boat'?

sinking both boats

When you insult or otherwise harm yourself to insult or otherwise harm another.

Guy 1: I bet I've had 20 times as many girlfriends as you
Guy 2: 0 girlfriends times 20 equals 0 girlfriends, idiot
(Guy 2 is sinking both boats)