I wondered about the same thing, Richard.
At the Whitewater Rescue Technician course in Jackson Hole in April, we spent a lot of time in the Snake River doing swim rescues, river crossings, and being the victim for rescues. I was toasty in a leaky rental dry suit. The guy wearing an Alpacka dry suit was also fine. I had a wool baselayer and two layers of fleece (on my torso) on under my dry suit. I don't know what the guy in the Alpacka had on.