Have a large bowl full of water set on a table in the room. In the bowl place a small plastic boat. If you don't have a boat, a Styrofoam bowl or other suitable container can be used. Next to the large bowl have a smaller bowl full of marbles. You could also use small rocks, or other suitable small heavy objects.
The object is to guess how many marbles (rocks, etc.) it will take to sink the ship. Have pieces of paper and pencils available for people to record their guesses. After everyone has had a chance to guess, gather everyone around and put the marbles into the ship one at a time. Keep count and determine how many it took to sink the ship. Determine whose guess was the closest and award an appropriate prize.