Hi
When all the drama started with the baby bottles a couple of years ago, there was a lot of info on the internet about which "numbers" were safe. Plastic bottles have a triangle (usually) and a number on the bottom of them and there was information about which numbers corresponded to BPA free. I would suggest that if the adult water bottle is not labeled as BPA free, that you go by the numbers (and do a google search to check which numbers you should avoid).
As I recall, the chance of BPA leeching is higher if the contents of the bottle are hot (as is the case for babies formula) compared with cold drinks - depending on whether you are expecting to carry cold or hot water in your bottle.
SB2