In nature, the beds of shallow streams and rivers are warmer than the water.
So why is it when I put my feet into the sand/gravel, mud, the temperature is warmer than the water?
You claim the temp is warmer below the sand like with cables, not at the surface(and this is only true during the day due to solar radiation).
Where aquatic plants exists and have been there for long time frames, the temps tend to be equal. Where plants are seasonal, the temps tend to be warmer only at the surface if it's shallow, but cooler underneath.
This is due to the sun warming BOTH the water and surfaces, while the sediment below where the roots are remains cooler.
If you want to debate this, please go and test the temps at different at different depths where the plants are in natural systems. Unless you have geothermal heating, this is not a likely situation(heating from below).
In lakes and very slow moving water, the differences are even more dramatic.
Lakes always(99.9% of the time) have
cooler water lower in depth, and it can be quite large.
Plants live in lakes, swamps, ditches, marshes also, perhaps more species than flowing water at least part of the season. Rivers/streams will be closer in temp with the water and sediments.
A more dramatic and obvious example: Why is it that the sand on a beach is hot on top, yet cool underneath?
This is opposite to what you are saying.
Regards,
Tom Barr