The call button is put there for a reason. Whether or not it makes the FA mad, makes her give you a bad look or not, it is there to call for attention. It is not a 911 number type deal where you only use it in emergencies. You press it to get assistance. If the airlines didn't want you to do it then it wouldn't be there or they would warn you on when to use it. Press it when you need to. Also, when you're sitting in F, the FA is supposed to be at your beck and call within reason. Certainly, asking for more water is well within what is considered reasonable. Don't beat yourself up over it.