I have a D9X, which is a tiny bit more heavy duty than the D9. I have only had it out on one warm weather boat trip - a weekend of diving down in NC back in September. I thought it was pretty comfortable during the SI's. WAY WAY more comfortable than my Bare XCS2. I was fine to keep it on, top still donned but unzipped, if I stayed in the shade. If I went up on the top deck in the sun, I would take the top off or take off the whole suit (which, by the way, is a lot easier than in a wetsuit).
What did you wear for undergarments?