In college, we did a number of pretty classic things.
We took the Company Officer's office and moved it into the nearby head (bathroom, for you civilian types). He was cool enough to operate out of the head for a whole day before he insisted we move it back.
Showers were always fun. Unscrew the drain, roll up a pair of black socks, and shove them into the space. Screw the drain back in real tight.
It's almost unnoticeable, unless you're actually looking for it. You get quite a ways into a good shower before you notice that you're not draining at all.
More shower fun -- our showers were little enclose marble stalls. If you were savvy, you could plug the drain pretty effectively, then use the shower curtain and a heap of duct tape to make a waist-high wall. Chop up a few bags of hot dogs and toss them in, then start the water running. If it overflowed, there was a drain out in front of the shower, so it wasn't a desperate problem. However, there's no good way to drain it down the shower drain, especially once it's full.
The other big classic was newspaper. Everybody had a subscription to the newspaper, as we were all required to be up on current events, which left us with a lot of old newspapers. There's nothing more fun than watching your upperclass come back to the dorm on Sunday evening to discover that his room is full to the eyeballs with crumpled newspapers.
If you were particularly cruel, you'd also tear out pages from skin magazines and wax them into the tile deck. That tended to be a real bear to get removed, and I was never known for doing that.
Oh -- the drawers in our desks were relatively water sealed. So were our bunks -- the space where the mattress sits would hold several solid gallons of ketchup. Don't ask me how I know that.
Missing mouse balls and frozen pillows are also classic. Onboard ship, you've got full-fledged deep freezers, so taking a normal pillow and making it a solid block of ice only takes a couple of hours. (smirk)
Ah, the good old days -- duct-taping coffee cups into the overheads, tying knots in the legs of people's coveralls moments before drill periods, and flipping light switches coincident with electric plant shifts; I can hardly wait to go back to a boat.
Dan