Muscle is heavier per unit volume than fat. According to Scientific American, fat floats in water, while muscle tends to sink because it is heavier.
Adipose tissue, commonly called fat, has a density of approximately 0.9 grams per milliliter. Muscle has a density of 1.06 grams per milliliter. The difference in density between muscle and fat tissue means that equal volumes of each weigh different amounts. Therefore, muscle tissue is heavier than an equal amount of fat tissue. As water has a density of 1 gram per milliliter, muscle tissue sinks when submerged, while less-dense fat floats. Athletes with very little body fat have to swim harder to stay afloat.