That's an interesting question. I don't have a lot of expertise on this topic, but my understanding is that tropical seas have some areas around reefs that are teeming with life and others that are relatively barren? So choosing a barren patch would seem the sensible option.

But I wonder. Even along reefs, there is this unfolding crisis of coral bleaching driven at least in part by global warming. Could a limited number of panels deployed in a loose fashion provide some shade and keep coastal waters cooler? Could this actually be beneficial to equatorial corals if done with care? There are a lot of assumptions here, though, and I wouldn't want to jump to conclusions.