We did sail DCL to Alaska out of Vancouver during the inaugural season. We thoroughly enjoyed Alaska and DCL but had our share of issues too. If you are going to Alaska give serious consideration to the Princess cruises that sail between Denali and Vancouver. They give you much more Alaska, including Glacier Bay.
We sailed with a Navigators Verandah and would definitely recommend it if you can swing it with the budget. Even if it means stretching the budget a bit. We found that being able to go out on the Verandah provided more shelter from the wind and rain while still providing an unobstructed view of the grandeur that is Alaska. We stayed warmer and didn't fight the crowds out on the deck along the rail. Tracey Arm day was especially great to have the Verandah because that is all day of sights alongside the ship. You go in, turn around and then go back out so you see both sides of the Fjord without having to cross the deck.
As for what to bring. Bring layers. Go to a sporting goods store if you don't have high performance fabrics for the undergarment layer. You'll be glad you did. That first layer is a wicking layer that keeps the moisture from direct contact with the skin. That alone will keep you warmer. The next layer is more typical clothing over that wicking layer, but if it is cotton, be prepared to change it out as it could become damp and cotton just doesn't dry well. The next layer is a nice fleece layer with the outer layer being a waterproof shell. You can add or remove any of the outer layers as needed for warmth. The outer shell protects from wind as well as rain and helps create a convection effect to keep you warm.
Bring gloves. People forget this but your hands do get cold at times, even on the ship. I like the kind that are not too think and restrictive so I can still use my hands. You don't need snow pants or anything like that, but some sort of long underwear (see above comments about the wicking layer) and jeans or a similar heavy pant. You may also want to consider layered socks as well. Do not do cotton socks if you are going to be doing any excursions that include hiking or long walks. Your feet will sweat, the socks will become wet and your feet will become cold, even with multiple layers. A thin wicking layer followed by a thicker outer wool and then a sturdy boot.
We brought more shoes to Alaska than any other cruise. Boots, athletic shoes, slippers,
crocs (for the pool area) and dress shoes (for formal night). No need for water shoes.
Don't forget some sort of knit cap for the head. Heat escapes from there faster than anywhere.
I've probably made you feel cold. It isn't really that cold. We went in early August and for the most part temperatures were in the high 60s with occasional rain. It just feels cold. It can get much cooler in the evening but I don't think it dipped below the 50s when we were there. The ship is moving though and it does create a wind chill so having some cold weather gear can make you feel more comfortable.
Bring a good camera! I bought my first Digital SLR before the trip and took some time learn how to use it. I still marvel at the photos I was able to take.