Alternatively, all the heliostats could be placed on a floating island as well with any concentrators and the whole setup could be rotated as a whole...
One possible advantage I am having in mind putting your solar furnace on water is that you could put some stirling engine in the focal point...
About the steering issue of large Fresnel structures, The old lighthouses, bearing Fresnel lenses weighing up to 3 metric tons were able to be rotate by the push of a finger(!) because they where floating in a mercury pond.
Likewise one could envision pressurized greenhouse like Fresnel...
I think the notion of a liquid lens is very interesting...
The water can be replaced with alcohol, to avoid algae and has lower density than water, Lavoisier used a giant alcohol lens in the 18th century to melt platinum.
People have mentioned infavourable span to weight ratio for giant...
