I hope I can allay your fears by saying that we we were on the Mercury Sept. 8 - 15, 2006 and sailed to Alaska from Seattle. The ship was in excellent condition, the staff was outstanding, the food and service were above expectations. We had beautiful weather the entire week, with the one exception being Juneau - but then I think it rains in Juneau every day of the year practically! The temperature was in the 60's and even hit 70 in Ketchikan. You will need warm clothes for the day you visit the glacier as it is VERY chilly that early in the morning and they get close to the glacier.
There is lots of deck space for walking and the space at the rear of decks 10 and 11 was our favorite. However, the Mercury just had an overhaul in California in April and added veranda staterooms in the aft section - which I believe wiped out the open deck area on 10. They also replaced all the bedding (I thought our beds were great), carpet in the casino, main dining room, hallways and grand foyer. I'm sure there were other re-do's but can't remember what they all were.
There was a great naturalist on board - Brent Nixon - and you will find that there are no obnoxious announcements being broadcast constantly. The only days we actually heard anyone on the PA system was for the lifejacket drill - day of embarkation, the morning we were at Hubbard Glacier, and the morning we arrived back in Seattle.
The scenery is outstanding! I will be very surprised if you do not enjoy your cruise on Celebrity Mercury. Hope to hear your comments once you return.