I'm quite sure that the hotel will be in the Anchorage area, then on the day the cruise leaves, they have let you know what time to meet and they will take you to the ship in Seward, somewhere around a three-hour drive, and absolutely beautiful!
Bus from Vancouver to Seward would be long for sure. . Don't worry, Princess land operation, based on our experience with them, is extremely well organized. They should meet you at the airport in Anchorage, take you to the hotel and explain your itinerary for the following day. The long daylight hours in Anchorage will give you time to see some of this interesting city before you have to leave for Seward. Route to Seward, part of which is along Turnagain Arm, is very scenic and the motor coaches are modern and comfortable. If the weather is clear you will have some great views along the way. Enjoy and expect to want to return again to Alaska when your cruise is over.