That's fine, because after you've unpacked you won't even see your bags again until it's time to disembark.
Most cruise lines issue a list of suggested clothing for time on board and ashore.
Layers are always a good idea. For an Alaska cruise, a good jacket, hat and gloves are a must, even in August. You'll be also be glad to have a water bottle that you can slip into your day pack. Good walking shoes, broken in before the trip, are best for shore excursions, whether you'll be strolling on cobblestones or clambering over Maya ruins. And don't forget your camera and film-and a compact folding umbrella, just in case.