It will sink. Give each team a fist-sized clay ball. Give each team a fist-sized clay ball. Give each team 20 pennies and challenge them to change their boats so they'll hold the most pennies. Make the boat bigger and the sides higher, to push more water out of the way.
Weight is only part of it.
Don't expect neat answers from kids on why certain objects float and others sink. They'll learn a lot by manipulating various objects. Drop several sinkers into the bucket. Show children Ping-Pong balls and marbles. Drop them into bucket.