What are the limits of ground effect over water on an airliner plane at cruising speeds?
Ground effect only comes into play within about 1 wingspan’s height above a surface and it is largely unnoticeable until approx 1/4 to 1/10 of a wingspan above the surface.

While it is true that ground effect greatly increases the lift to drag ratio of the wings, it does not reduce parasite drag nor does it account for the fact that it is more efficient to fly at higher altitudes where the atmosphere is much thinner for greater speeds per unit of fuel consumed.  Flying low to the ground is dangerous as there is less margin for error if something goes wrong and would limits operations to day VFR conditions.

Transport airplanes have been test flown and certain flight profiles have already been determined to offer the greatest range.  These occur at altitude and on reduced power settings.

Heavy sea transports which to use wing in ground effect for lift  have been proposed in the past, but been found to be in efficient compared to existing jetliner designs. WIG craft have been used for short transport missions like island hopping, where that kind of a craft would be faster than a ship, and more efficient than a light airplane.