This is an old favorite story. I think I got my version of it from Garrison Keillor.
There were once two good friends who went rented a boat and went fishing.
They spent an enjoyable afternoon and caught some fine fish.
When the sun began to set, and the fishermen were starting to get a little sleepy, they brought out the thermos of coffee, and added its contents to the cups of whiskey they'd been drinking all day as they fished.
Alas, the whiskey had a head start on the coffee, and one of the fishermen accidentally dropped the thermos over the side of the boat.
"You fool," said his friend, "that was my wife's thermos, and she'll be plenty mad to learn that we've lost it. The water is too dark and cold tonight, and we'll never find this spot again in the morning!"
"Don't worry," said the buddy, taking out a pocketknife. "I'll mark the side of the boat, so we'll be able to tell exactly where we dropped it."
"That's no good!" said his friend. "Suppose tomorrow they give us a different boat?"