I'm not a doctor but in my amateur opinion I'd say that water makes for a more than adequate solution to any 'dry mouth' issues you may experience.
In fact, it's not just mouths that can be made less dry by water.
I don't want to blind you with science so let's just say that water is one of the wettest things in the world. It's brilliant at being wet and can be used to make almost anything less dry.
It definitely works with mouths. Well it certainly does with mine. In fact my mouth was feeling a bit dry only two or three minutes ago. I drank a glass of water. Problem solved.
Of course water isn't always available in a big city like London so it's worth pausing to consider how I might deal with a dry mouth if I found myself unable to locate drinking water. If only someone could invent a dry mouth spray of some kind.
Of course while an amateur like me is capable of coming up with the glass-of-water solution it would need an expert to come up with a dry mouth spray... after all, it's not like it's going to be mainly water now is it?
Oh. It is.