Forces Acting on a Swimmer
http://www.feelforthewater.com/2013/08/whats-right-amount-of-rotation-in-your.html
Shown above is the force diagram of the forces acting on a swimmer while swimming. Gravity pulls downward on the swimmer while buoyancy keeps the swimmer at the top of the water. The thrust created by the swimmer propels them forward while drag from the swimmers shape, swim suit, cap, and goggles pulls the swimmer backward.
http://www.secsportsfan.com/sec-swimming-and-diving.html
When a swimmer is preforming a dive, or start, off the blocks the forces are slightly different than that of a swimmer already in the water. The swimmer pushing off the blocks creates thrust in the x-direction. Gravity pulls down on the swimmer and brings the swimmer while in midair toward the surface of the water. Drag pulls the swimmer in the opposite direction from the direction of thrust.