mizu ga nomitai n desu ga

mizu ga nomitai n desu ga mizu ga nomitai desu Do I understand this well: here is only a difference in politeness? I'm just studying since a couple of weeks and I have to go fast: I'm going to Japan on a solo trip next november! Thank you for responding.

Yes think of "mizu ga nomitai~n desu ga" as being more polite than "mizu ga nomitai desu". The first one you use when you expect a response/ are asking for something/ telling someone what you want to do in a formal situation. The second one is more of a protestation. Lesson 2.8 has a good explanation on how the ~n desu ga construction is used, (~n desu ga being a shortened form of no desu ga) I also posted an explanation in your thread about "shitai n desu ga".