This ground effect you speak of is an increase of lift within about a wings length of the ground caused by the disruption of wingtip vortices (or induced drag)... for you non aviation people, aircraft have less drag near the ground increasing lift and thus performance. I would assume the biggest challenge for these pilots would be to not dip a wheel below the water surface (for obvious reasons!). I believe (though I have no knowledge of seaplanes or flying over water) that being over a calm lake there would be nothing disrupting the airflow and thus it would be pretty smooth over the water. regardless, I myself would never attempt something that insane, damn!!!