You swim faster slightly underwater because you minimize surface resistance, which is a significant drag force when swimming on the surface.
Swimming involves an interaction between the swimmer and the water. When a swimmer propels themselves, they use their arms and legs. However, the water slows them down due to drag. This drag is composed of several factors, including:
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Surface Drag (Wave Drag): This is the major drag force acting on a swimmer at the surface. It is caused by the creation of waves as the swimmer moves. Swimming underwater eliminates wave drag almost entirely as the body is no longer interacting with the surface.
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Form Drag (Pressure Drag): This is caused by the shape of the swimmer's body and the pressure difference between the front and back of the swimmer as they move through the water. While this still exists underwater, the elimination of wave drag generally results in a faster overall speed for a short distance.
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Frictional Drag (Skin Friction): This is due to the friction between the swimmer's skin (or swimsuit) and the water. This type of drag is still present whether the swimmer is on the surface or underwater.
While underwater, swimmers are not subject to wave drag. This allows for a faster, more streamlined movement, at least for a short period. However, regulations in swimming competitions limit the distance a swimmer can travel underwater at the start and after each turn, because after a certain distance the lack of available oxygen makes it unsustainable and counterproductive.
