No you don't have to disembark, but you may have to go through some kind of Customs or Immigration, I'm not sure.
You will receive a letter in your cabin outlining what the procedure is & a transit pass so that you can get off the ship and spend the day at your leisure or you can just stay on the ship.
If you are staying in the same cabin you don't have to do anything about your luggage - no packing, unpacking - just enjoy your day.
Enjoy Alaska - it's beautiful.