the difference of the resulting turbulence from the wave bounced back from the bottom surface at 2m here and from the more traditional 3m is a big deal. The water is pushed by the swimmer's hands with the speed of something on the scale of 2 meters per second, so, as the swimmer moves forward, that turbulent movement of the water reflected by the pool bottom may as well come behind the legs in the 3m depth case while in the 2m depth case it would catch the legs decreasing the efficiency of their movement.
