Princess, NCL, HAL all go to Glacier bay. Not sure of others that might. But realize that not every ship on those lines goes to the Bay either and some ships don't go every cruise. Glacier Bay is awsome and worth the extra research you'll do to find the right cruise.
Weather's always iffy in Alaska. There's always the risk of rain. Most of the land by the inside passage is rain forest.
June, July, August for the better chance of good weather. May and September for the best pricing in most cases. Funny how it works out that way. ; ^ )
But remember you can get great weather in May and lousy in August too. It changes from day to day in some cases.
You're making a good choice in choosing an Alaskan cruise....believe us.
And IMHO........go for the balcony.