I live in the Caribbean, its HOT here all the time. Our winter got us down to 72f ONE night. I'm finding more and more people in Cayman with ponds. I'm wondering if fish just adapt to the environment. My ponds deepest side is 3 feet. I have lots of cover, about 1/3 of the pond is covered with floating plants (netted water hyacinth), they do spend the hottest part of the day under the plants. The pond is shaded a good bit in the morning and afternoon, but mid day sun beats down on it. My fish are totally fine. My neighbors have had their pond for years, theirs is smaller than mine and the koi does just fine and are quite big.
I used local plants in the pond, so the heat doesn't bother them. I have two water lilies that are not native and they seem to be very happy with blooms every few days.
So even though I am fairly new to the pond scene, I would say floating plants or other floating shade and some plants over hanging the pond (ferns or something like that), maybe some hiding spots for the fish and you should be fine.
I will tell you something weird that the guy in the pet store told me, he is from the Philippines. He said to make sure the pond was filled to the top of the over flow whenever it rains. He said the temperatures don't mix and creates some sort of barrier and can block the oxygen. That may be a problem for you in rainy season (if its actually true, I don't know).
