A number of years ago we took the ferry from Skagway to Bellingham returning from a motorcycle trip to Prudhoe Bay. Great trip BTW. We had an outside room, with large window, private bath with private shower (all upgrades), and it still cost more to transport two dirt bikes than the room cost (10' minimum - then pay by the foot thereafter). Check out the prices on their website - it's not cheap. Yes, you can sleep on the solarium on lounge chairs (first come first served), pitch a tent on the bow (if room available), or sleep in an enclosed area (must clear out during the day - to where ???), but remember you are in the weather on deck and the solarium roof leaks BAD. So choose your adventure wisely. If you are not a young backpacker, you might want to get a room (the 'campers' really looked tired after 4 days w/ weather). It is an excellent scenic trip though - very close to land (both sides) much of the way. The ferry takes the same route as the cruise ships. We had access to the vehicles at every port the ship stopped at - NOT ever while underway (you cannot sleep on your boat). There were many dogs in vehicles that needed 'walking'. The ship bar was open while we were onboard. Restaurant (& snack bar) were OK - bring special needs. Don't be late returning from a port - you'll be left behind as your vehicle sails off without you. LOL (OK so we cut it way too close once in Ketchican - don't ask, but we had fun 😀 ). We thoroughly enjoyed this leg of our trip. I highly recommend it. They gave an excellent narrative each morning, too (and I'm not really into that kinda thing). Have fun and enjoy your trip.
Todd
EDIT: You NEED reservations - book early = NOW!.