A solar pond is simply a means of using water to store solar heat radiation. The heat absorption capacity is enhanced by various means such as (1) a black cover for the bottom of the pond, (2) a layer of oil on the surface to prevent heat loss, or (3) adding copper sulphate to the water which absorbs radiation near infra-red (and prevents algae growth). The stored heat can be used for space heating or some light industry process.

Now to the question. My guess is that although solar ponds received some attention in the 1970s when we thought we were immediately running out of low-cost fossil fuels, they have not increased much in use. I certainly have not seen one, and I have kept track of a multiplicity of diverse energy technologies and processes around the planet over the last two decades. So the qualified answer is that solar ponds are not widely used, probably because conventional energy is still relatively cheap.

A website on solar ponds is run by the University of Texas at El Paso.