I usually play it by ear. If the person is a parent and their kid is underfoot... I know it can get dicey when you have guests and a kid to keep an eye on, so I often say no thanks. If there are two parents/people there though, then I might say yes to water. Or if they have a pot of coffee already made, then I'll say yes to that. Basically, whatever's easiest for the host... not looking to rock the boat.
When you're at someone's house and they offer to get you water/coffee/etc., how often do you accept?