We already know that as the body moves through the water, it disrupts the flow of water. As the body moves forward, water is given a momentum backwards and travels until the velocity is 0. The water behind the swimmer follows the motion of the swimmer and creates drag. If two people are swimming in a straight line with one in front of the other, the person in the back is being pulled behind the swimmer in the front by a small drag force. As the swimmer in the back slides their hand into the water for the catch, they are placing their hand into water that already has a momentum. For this reason the person in the back does not have to work as hard to travel the same distance.
The swimming pool has floating lane lines that typically divide the pool into
six swimming lanes. Within each lane, the motion of swimming is counterclockwise
(i.e. swim down on the right and return on the left). These floating lines keep
waves to a minimum by knocking them down. They also minimize the momentum of
a body of water after is has been pushed backwards. The water vortex breakup
when they come into contact with the lines.