All would help keep the water cool and all have other drawbacks as well...copper and pumping cost big bucks...if wanting some geo-thermal effect, go with the tubingmade for it..still cheaper to re-stock annually.
Lillypads will do minimal shading as the leaf itself will absorb lots of heat to be transferred into the water as well as making fishing difficult or a variety being highly invasive.
an awning may help shade and keep leaves out, but be a whole lotta work to keep maintained and cleaned.
