We were on the May 17-24 cruise last year.
On the ship, think of it like a hotel. Jeans, light-weight top, whatever you would normally wear inside somewhere. But, then someone says, "Whales!" And you run outside, but it's chilly. Put sweatshirt. Go back inside for bingo, sweatshirt off. Then you decide to go up on deck for an extended time. The ship is moving and it's chilly with the breeze from the motion. If we were on deck for a while, sweatshirt and scarf or just warmer jacket.
On Tracy Arm day, we were outside on the deck all day. It was chilly. Warm winter jacket.
In port, it was a sweatshirt over long sleeved tshirt. However, when we did the dog sled excusion, winter jacket because once those dogs started running, and we were being pulled, wind affect = cold.
Train excursion: inside the train, long sleeved tshirt, maybe sweatshirt. When out on the platform with the train moving, jacket.
On and the jacuzzi was very nice. When getting out of the jacuzzi, very cold! They had blankets on deck on every chair.
Bottom line: Inside, dress like any hotel, but remember if you want to go outside, it's chilly because the ship is moving. At ports, lightweight jacket, but if moving fast, heavy jacket because of the cold air.
We were VERY lucky. We had no rain all week.
Hope this helps,
Dreams