Much like a blumpkin
(a man receiving a blow job while sitting upon a toilet, passing feces.) A snorkel is when a man has vaginal sex with a female, while she herself is using the toilet, passing feces. The man's penis is submerged below the toilet seat level, with the rest of his body on the outside of the toilet.
Mr. Testy: Hey Timmy, why do you keep scratching your penis?
Timmy: Well Mr. Testy, last night my girlfriend and I were horny, but she had to take a poop, so to save time, I had sex with her while she was pooping. It's called a snorkel sir.
Mr. Testy: Ah, a snorkel, brilliant!