Yes we went to Alaska with Celebrity Mercury this August, superb in every way. We sailed from Vancouver having flown with BA the previous day. It is best to take layers as the weather can be quite warm but also cold and wet. Layers can then be removed or added. We took trousers, t shirts (long and short sleeved)skirts, formal dresses, fleeces and hooded jackets, also gloves. If you go to Hubbard Glacier you may be as lucky as we were and have a bright sunny day but crisp. Celebrity were first class from meeting us at Vancouver airport, to escorting us to our hotel and then to the port next morning for boarding, the cruise itself and then back to Vancouver and home. You'll have a super time , enjoy and I wish I was going again.
Even San Francisco can be chilly in April so take some warm clothes and you will need them as you go north. You can get some warm days but you are far more likely to get rain or mist, certainly once you are past Victoria. It is however a good time to go with the routes just opening up to tourists and after the winter the locals will be glad to see a renewed source of revenue. Have agood trip....Nei.
Layers is a great suggestion. for our alaska cruise in mid-may '06 on the Summit we took anoraks, hats, scarves & gloves & pullovers, but at the three ports, skagway, ketchikan & juneau the weather was glorious & as we walked around, just peeled the layers off, bit by bit. (we were told that the previous two weeks it had rained all day). I hope that Celebrity are still doing the pea soup in a sourdough bowl!