I love sailing through the fjords of Milford! I’ve done this a few times now, and there is always something new and a different kind of light.
I think a lot of people are disappointed if they get to Milford Sound and it is raining. I would not feel that way if I were you because things can perhaps be at their most dramatic! There can be a ton of little waterfalls and crazy clouds even in the middle of the rain. So don’t let the rain dissuade you from taking the boat out and about!