I suspect (keyword - suspect) that any submerged shade structure will have little to no affect on actual water temps. Anything that shades the bottom will reduce the temp of sallow bottoms and give the underlying inhabitants some relief from the sun's radiant heat, but the structure will still heat up itself hence heat the water around it as well. The only shade structure that would/could noticeably reduce water temps would have to be above the water surface.

Reflective submerged structure could reflect some light/heat back out of a pond, but, as we all know, submerged structures all turn grungy after a short period and the reflectiveness would soon disappear and become an absorber of the light's heat.

All in all, I believe the water temps would remain unaffected, but shade structure can certainly produce areas where the pond creatures can escape the radiant heat of the sun which is good for the dog days of summer in the warmer climates. Even though they can not escape the water temps at their preferred DO levels...they can get out of the sun and enjoy the shade.

Fish on!,
Noel