I have dived in Thailand, both Gulf of Thailand and Andaman Sea side, in the months of February, March, April, September and October, always in a 3mm short wetsuit, and never really had any problems with water temperature. However, every now and then I get itchy red spots where my arms and legs aren't covered by the wetsuit - guessing tiny jellyfish stings, or something of that nature; nothing serious but pretty annoying, so I'm thinking about getting a full-length 1.5mm or 3mm before my next trip. Definitely not 5mm though, that's way overkill.