As jno says, during an Alaskan summer it rains! The week we did it in 2005 we had no rain at all (and were told we were really lucky!). The cruise before us it had rained every day, so I'm afraid its a bit of a lottery. Take clothes for a British spring. It won't be scorching hot, but hopefully pleasant. The scenery is breathtaking (hope you have got an outside/balcony cabin). We saw Killer and Humpback whales and loads of bald eagles. Juneau is a seasonal jewllery shop, Skagway was good (recommend the Yukon and White Pass railway), Ketchikan was OK, went to the Rainforest Park (there is now a zip line trip there), and the captain sailed the ship within 200yds of the Hubbard glacier...which you can hear creaking all the time. If you are starting/finishing in Vancouver, I recommend stay of about 3 days. Its a great place. You will find loads of other info at
http://www.cruisecritic.com/
Have fun!