Bottled water is pennies, 10baht, and everyone drinks loads of water because of the heat.
I can't honestly say I have ever had salad in Patong, or seen any come to that, but that is more likely to be my culinary habits than lack of produce. Most of the eating places seem fairly clued up though, so I would not worry too much.
A pharmacist once told me not to eat food that has been rained on and I am religous about that now, but water and hygiene are not much of an issue generally. I'm never far away from a dose of immodium though, just in case.
