First stop: cruisecritic.com. Their articles and their message board are absolutely invaluable. They are to cruising as the disboards are for disney parks.
We had our honeymoon to Alaska from Vancouver on
RCCL. Booked directly through RCCL. Especially with RCCL, where you can't get true discounts from travel agencies (TAs can give you perks like paying your tips, or giving you a bottle of wine....they can also book blocks of rooms and get a discount that way, but you lose a LOT of flexibility if you're inside a block), it's not really worth it to find a trustworthy cruise
Travel Agent, if you don't already have one.
I'm not sure the cruising season really starts in spring...it's generally over the summer. AK has to warm up a bit! We had weather all over the place. Freezing on the bow while heading into the glacier area (Hubbard Glacier, maybe?), sweating while walking around Juneau, jackets without quilty liners in Skagway. Our cruise was the second *last* of the season on RCCL, just before Labor Day weekend, and it's possible end-of-spring and closing-in-on-fall have around the same temperatures.
I highly recommend getting 3 in 1 (or 4 in 1) jackets whenever they are in stores. We had to scour the earth to find them during the summer! Finally found an outlet selling London Fog and their sister line of coats. Nowadays
amazon sells clothes (not in '03, at least not to a big extent), but with a coat like that I like to try it on.

You want the warmth but also the flexibility.
That's just my helpful hint b/c it was the biggest pain of the whole planning experience for the cruise.
If you do an Inside Passage cruise, the water it's really all that rough. But if you leave the IP to go out to the glacier, that gets you into rougher, more open-ocean type of waters, and that can get rough! I woke up at 2am thinking extremely negative thoughts about what was happening, b/c the waters changed SO much. DH used to work on oil tankers that went from CA to Valdez AK and back, so he was sleeping perfectly through the night and was of NO help, LOL. But it was scary for me, although entirely normal and FINE. Another helpful hint, LOL.