I've dived my Freedom plate with a T-shirt in the Philippines and Cozumel. I have also made a couple of dives without a shirt with no problems.
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However one question I still have is above the level of comfort wearing one without a wetsuit, or maybe at most a rash guard. I am nearly exclusively a warm weather diver, with the exception of an occasional cold'ish water dive (Greece for example). Are these backplates comfortable on the back against bare skin? I would assume I'd want some bouyancy neutral padding on the plate?
I dive a BP&W and was in warm water in the Florida Keys about a year ago with just a swimsuit. It was comfortable.
The only situation where it is not comfortable is when carrying the full kit (BP&W, tank, regs) a considerable distance topside, as is sometimes necessary for shore diving. The straps dig in. Usually I just put a towel over my shoulders first to provide some protection.