I got cave certified one January in a 7 mm suit. I will never forget the misery of stripping off my suit in 50-degree weather to change for a lunch, a gas fill, and a de-brief, only to have to shiver and shake my way back into my wet skin and wetsuit for the second dive after lunch. My husband and instructor were toasty in their drysuits - they stripped off to their long underwear, and that was that. I started shivering as soon as I got back into the water - they didn't. Right then and there, I knew I would have to invest in a drysuit.
Don't get me wrong - a drysuit presents its own issues. The upfront cost, obviously, and ongoing maintenance (seals, etc). Reconfiguring your gear (BC, even fins) to accommodate the suit and the different buoyancy characteristics. The humiliation of the inevitable feet-first ascent while you're learning to drive it. The discomfort of roasting in hot weather topside. Figuring out the whole pee-thing. (Blokes have it much easier!) The frustration of the occasional leak or even outright flood. Thing is, is that drysuits open doors that will be forever closed if you keep diving wet. Cave dives lasting hours in cold water can be managed in a drysuit (although one may have to up the ante re: thicker underwear or even heated vests), but in a wetsuit: not so much. There are always exceptions - I know a few folks here in the Northeast that dove wet for ages, and people in cave country today that do staggering dives on OC in a wetsuit, albeit with heated packs to help them - but for most people, dry is the way to go.