If the focal point is deliberately placed behind or into the object you are trying to heat, the surface area being heated increases, because the beam of light is conical, with the closed end being the focus.
A fresnel lens would still gather the same amount of light whether it is placed a yard or an eighth of an inch from the water's surface.
So theoretically, you could choose a dark object to put into the body of water, say, a dark rock, and then adjust the distance of the lens from the rock such that the beam where it hits the rock covers its whole surface area.
Also, there are plastic fresnel lenses made into sheets for the purpose of enlarging print.
Kevin EarthSoul wrote:Redirecting sunlight coming in at a low angle down into the water by the use of reflectors is one thing to consider