"What's pulling you down?" (The force of gravity.)
"What helps push you up?" (The force of the water, also known as the buoyant force.)
"That way Mommy doesn't have to work against gravity to hold you up all by myself. The water helps me."
This conversation is leading toward the idea of net force, the totaling up of forces on a body that determines its acceleration. It was also a satisfactory answer for my four-year-old on a beautiful day.
Related post: Pre-school science: Ducky in the Bathtub