My Doc suggested I carry some imodium with me, just in case it was required for the flight home & to avoid looking all furtive when lined up in customs. Though I suppose that if customs think your behaviour makes you look suspicious & they want to do an internal, then I guess the mess could be on their hands - literally
That said I've had 2 trips & no probs. When buying water I was told to make sure the seal wasn't already broken, as this could suggest the bottle has been refilled with tap water