Searching for posts regarding "Sitka" I came upon this thread, so I thought I'd reply even tho' it's a few months old...
I'm heading up to Alaska for my second liveaboard trip next month. (I did the Nautilus Explorer in July 2006 and we had a GREAT time!) I've done several other trips on the Explorer (Vancouver Island and 2 trips to Mexico) and this will be my first trip on the Swell.
I do most of my diving in Monterey, CA, so I'm already used to bad viz and cold water and I'm very comfortable in my drysuit. Last Saturday's dive here was 46 degrees! IIRC, during the 2006 Alaska trip, the water temps were 46 - 48 degrees on most dives, but we did have one dive where my computer said 43 degrees. And I think it was 38 degrees the day we snorkeled amongst the icebergs.
What impressed me most about Alaska diving was the size and sheer numbers of the sea life. Vertical walls just completely covered with life. I think the juvenile sea lions up there are the size of the adults down here. And we saw some really impressive ling cod.
Plus, you can only do a couple of dives a day because you have to time them for the turning of the tides. So it's not like a warm water trip where you do 4-5 dives a day. But that leaves time for other activities. Like being on a mini cruise ship we had the opportunity for several "shore excursions" to check out various towns, go hiking, hang out in some hot springs, etc.
I would highly recommend diving in Alaska.