Along the lines of bobing for apples but a more extreme. Have the youth leader dress up as “Omar” the master of things that float (or not). Set up a table with a long table cloth over it so you can hide the items to be floated under it. Have a large, fairly deep, and preferably see-through bowl of water on the table. Under the table place a unique collection of things that will either float or not. We have used lettuce, smarties, onions, cat food, whipped cream, and so on.
Ask for several volunteers and line them up. Blindfold the first one and reveal the first item to the crowd asking for a vote on “will it float or not” without revealing what it is. Put it in the water and get the volunteer to bob for it and try to guess what it is. Points for getting it right and extra points for retrieving a sunken item. Continue with the next volunteer.
Very messy game but great opener for a party or youth week-end. Have towels standing by.