"Hot and humid" are two words that describe Thailand. I am not complaining-- just saying.
I don't easily get thirsty, but I seem to be drinking lots of water here than when I was in our home country. Just to be out in the sun for a few minutes could be so draining, that's why it would not be surprising to find people bringing water with them in their bags.
It would be helpful also to have a pair of sunglasses to beat the heat. Getting those cheap ones could really cause headache, so it pays to invest in good quality ones. Maui Jim Sunglasses really has chic and cool designs, and there's no question about its quality. So am I getting a pair soon? I'll try to save up for it, that's for sure!