(to put it delicately...) Don't ever drink tap water - bottled water is cheaply available everywhere, and should be provided free in hotel rooms. Think twice about ice in drinks, too. Elsewhere water is much cheaper from a supermarket than from a tourist-oriented stall. As long as it's in a sealed bottle, it doesn't much matter which brand you choose. Supermarkets (e.g. the ubiquitous 7-11 franchise, often open 24 hours) are a good source of cold drinks, ice-cream, toiletries and the like. Pharmacies sell basic remedies for most things - including Imoduim/Diocalm/Diareze, dehydration salts and the like if you're unfortunate enough to need them. Basically, unless you have special medical needs, there's no need to take anything special with you - except maybe suntan lotion, which can be quite expensive. The same goes for clothes.

Outside of Western-style hotels you are quite likely to meet the Thai-style squat toilet. Unlike the French hole-in-the-floor they are raised off the floor. Flushing may be by pulling a chain, or you may have to use a scoop to ladle water out of the tank beside the bowl. As in Greece, don't put paper down them - the Thai technique is to wash yourself with the dipper or the little shower head provided, and then use paper only for drying, and dispose of it in the bin provided. Toilet paper is not usually provided in these facilities: either take your own, or you may be able to buy a packet from the attendant or a vending machine. Don't be surprised, on visiting the Gents', to find the attendant massaging your shoulders. (If you're female, be very surprised!) Either let him or decline politely, but he expects a tip of around 10 baht.