A Shark Shit is the disgusting act of someone swimming at the beach that is too lazy to get out of the water and use the public restroom like a normal person, instead they take a shit right there and then in the ocean.
"Dude, its freakin cold out there and those restrooms dont got doors on the stalls; Fuck it! we shark shittin today!!!"
The act of giving someone fellatio, while submerged in a bath tub. Stipulations include, the giver wearing a snorkel, and the receiver defecating a floater so it bumps into the givers head, effectively acting as the shark.
Bill- "That was a great shit-shark! Hopefully we can do it again sometime?"
Sarah- "I don't think so, my head is bruising."