Person who takes a dump and doesn't flush it right away, but prefers to sit there, a scant few inches, above their sopping wet pile of stinking feces.
Public washroom, guy in stall #1 hears guy in stall #2 drop the splashbomb. In the next few seconds there is rancid odor, but no flushing!?! So, guy in stall #1 bangs on the wall, shouting...
derived from the words Turd and Submarine. A Turdmarine is created when someone accidentally shits themselves whilst taking a bath, and the piece of crap is left floating around in the water.