Cars go too. If you really want to travel north on a budget, you can bring your sleeping bag and sleep on the floors or set up a tent on deck.
Smaller ferries operate throughout Alaska.
No cross Gulf of Alaska ferry. These vessels carry cars, campers, etc. as well as passengers. At most stops there is time for passengers to explore the town before the ferry leaves for the next stop. This is the most economical way to travel around the region - many passengers bring tents to camp on deck, or just sleeping bags and pillows and 'crash' on the rugs, sofas, etc. Most vessels have cabins as well and all vessels offer meal service - usually cafeteria style.