I wore a 3mm shorty when we got certified in Hawaii in April, and had planned on buying just a shorty for warmth. I soon changed my mind! On our final certification dive, one of the people we were diving with (who was wearing a shorty) came up with about 50 sea urchin spines in her knee!! :11: She had apparently "brushed" the reef with her knee and was in a considerable amount of pain because of it.
Because of her, I decided to buy myself a 3mm full suit to protect against what's in the ocean, including jellies and fire coral when we go to the Caribbean later this year.