When filled or partly filled with air, tennis balls will float. However, some of the materials in them are heavier than water. If the air is removed, some tennis balls may sink in water, as do rubber tires.
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In water? Yes; water's reasonably dense compared to a lot of "household" objects. A football is even pumped full of air making the average density pretty low.
Try placing an inverted bucket into a large volume of water (eg bath) and sinking it. You have to push down pretty hard.