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Not only does it depend on your cold tolerance, imo, but it also depends on how much you plan to dive. If you're doing four hours of diving a day for five days, a skin is probably not enough (for me, even a 7mm is not enough). But if you're doing a handful of dives and don't get cold easily, a skin is probably plenty.
It's a pretty huge bummer if you plan to do a bunch of dives and either find that you can't or you're not enjoying them because you're cold. It's a lot easier to cool yourself off if you're too hot (by flushing water through your suit), but not a lot can be done about warming yourself up if you're too cold.