A watermelon floats because it is less dense than the liquid in which it floats. An object's density, relative to the liquid it's sitting in, is the property that determines whether an object floats or sinks. If an object is denser than a liquid, then the object sinks.
Density is calculated by measuring an object's mass relative to the object's volume. Mass refers to the quantity of matter which makes up an object. Volume measures how much space the object occupies. Two substances can take up the same amount of space; however, if one substance is a cube of fluffy marshmallow and the other is an equally sized cube of a heavy metal like lead, then their masses and, consequently, their densities are very different.
Watermelon floats in water because it is less dense than water. Within the watermelon's rind is an airy, juicy pulp with some seeds scattered throughout the fruit. A watermelon may feel heavy, but an equivalent volume of plain water would feel even heavier.