A man joins the navy and is shipped out immediately to an aircraft carrier in the middle of the Pacific Ocean. The captain is showing the new recruit around the ship, when the recruit asks the captain what the sailors do to satisfy their urges when they're at sea for so long.
"Let me show you," says the captain.

He takes the recruit down to the rear of the ship where there's a solitary barrel with a hole in it.

"This'll be the best sex you'll ever have. Go ahead and try it, and I'll give you some privacy."

The recruit doesn't quite believe it, but he decides to try it anyway. After he finishes up, the captain returns.

"Wow! That was the best sex I've ever had! I want to do it every day!"

"Fine. You can do it every day except for Thursday."

"Why not Thursday?"

"That's your day in the barrel."