I'm considering a Galapagos trip in September, but I don't like wearing lots of neoprene, and I'm not qualified for drysuits. Would it be reasonable to dive in 20-22C water in a full-length 3mm plus this thing or something similar?
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I have a Thermolution vest under my drysuit. They are made to be used with wetsuits, as well. Never had any issues and know many people who use them. Suggest you look into them.
We wore semidry suits on that trip and were comfortable, even hot for some of the dives.