My wife and I recently went on the Mercury northbound sailing from Vancouver to Seward. The weather was great and they even broke a high temperature record in Yakutat (Hubbard Glacier).
But even with the warm weather, it was basically too cold for most people to want to use the outdoor pools. Fortunately, Mercury has a pool with a retractable roof on the stern behind the Palm Springs Cafe. This covered pool was very popular with the 170 or so kids on the cruise.
On the Millenium class ships (Millenium, Infinity, Summit) there is no indoor pool available except the $20 per day Aqua Spa. The Aqua Spa is definately not a kids thing. Unfortunately, next year the Mercury will be replaced on the Vancouver-Seward route with the Summit. This will leave Celebrity customers without a covered pool option in Alaska.
We loved our Celebrity cruise and would recommend taking them to anyone with one exception. If you like swimming or are taking kids, don't cruise Alaska on a Millie class ship - you'll freeze!
You may want to read some of the other threads in this board. It seems that Celebrity has started charging everyone $15 per day for the Aqua Spa on at least some, if not all, Millie class ships.. (The exception to the rule is Suite passengers.) See the threads "Important Infinity/Summit Information", and "Charging for Spa&Pool on Infinity".