I used a drysuit in the Red Sea for a December trip a few years ago, no doubt in part thanks to Lynne's post. But it was mostly for protection against topside chill due to the wind on the RIB ride back to the liveaboard. I also use a drysuit in Mexican caves (water temps around 76F). But that is mostly for redundant buoyancy. I don't do 2-hour dives, but if I did, it would likely be in doubles with a drysuit.