How does momentum apply underwater?

Answer 1

Momentum is a property of matter which causes matter to maintain velocity (speed and direction) unless acted on by unbalanced forced. This applies anywhere, from in space, in the air, on the ground, under ground, or under water.

Once a swimmer is moving he/she has momentum. Momentum is a vector quantity is calculated by multiplying mass by velocity. When moving through a fluid, whether it be air or water, a body encounters resistance. This resistance causes momentum to be lost to the fluid around the body, and the body slows down. Normally a constant force such as a waving a fin, or spinning a propellor is needed to counter the loss of momentum. Aero/hydrodynamic shapes will lower resistance allowing the body to retain momentum for much longer, and reduce demand for a compensating source of power.