I think you are wise to fly in a day early. Our ship departed from Vancouver so I don't know the hotels near SeaTac.

As I mentioned, we cruised in mid-July, shortly after Glacier Bay was reopened as the seals' pups were maturing. Even though many had left, the ice was still covered with moms and pups. With binoculars, we got a good view of them which was a treat.

The only folks who had any problem with mosquitoes were the few who took the floatplane excursion to a lake lodge. We didn't encounter them on stops like Skagway, Juneau, Sitka, etc.