What the Heck is "Snorkeling" would someone please tell me!

Rep. Eric Massa stepped down from Congress on Friday but that hasn't stopped new information from coming out about his past behavior. In fact, it seems that his resignation came shortly after the House Ethics Committee were notified of Massa's behavior in the Navy. Massa's behavior in the Navy was not unknown to some in Washington.

An article in The Atlantic details some of this behavior but leaves out one critical piece of information: Would someone please explain to me what "snorkeling" is?

Clarke says that Massa's roommate, Tom Maxfield, was also assaulted. "Tom lived on upper bunk," Clarke say. "When you're on ship, you're almost exhausted 24-7. So a lot of times you sleep with your uniform on. Tom and Massa shared a stateroom together. Massa climbed up on the top of his bunk, which is hard to do--you never crawl up on somebody else's bunk. He wakes up to Massa undoing his pants trying to snorkel him."

Now let's be the first to define (invent) some other obscure and inappropriate naval behavior like:

Brass Monkey

Here's the official definition: A holder or storage rack in which cannon balls were stacked on a ship. Supposedly when the "monkey" with its stack of cannon balls became cold, the contraction of iron cannon balls led to the balls falling through or off the "monkey."

And how about a

Coxswain
Again, official definition here: The swain (boy servant) in charge of the small cock or cockboat that was kept aboard for the ship's captain and which was used to row him to and from the ship.