If you test positive on the ship, Princess will isolate you, and quarantine your companion. My wife tested positive on June 3, day before debarkation, and could not debark on June 4. They let me off the ship, although I think they were not supposed to. Since they kept her on the ship, the current plan is to debark her in Whittier on June 11, and complete the isolation in a hotel (Anchorage?) and fly back from there.
Majestic Princess crew has no idea what they are doing. They first thought she could stay in her cabin (balcony) for the return to Whitter, but ended up moving her on Sunday to the isolation floor (Dolphin deck). The best I can describe it is: The ship was in mothball for 1+ years due to covid. When taking out of mothballs, they freshened up the decks they were going to use (roughly 50% occupancy). Due to Vancouver hotels filling up with isolated passengers, they developed a protocol to isolate on the ship. They re-opened the deck, but did not freshen it up. They also don't have the staff (or are not willing to provide the staff) to take care of the passengers in isolation with any reasonable level of service.
So, your choice if you want Princess to pay for your isolation on a sub-standard cruise experience, or figure out something else. In my opinion, plan to get a transfer to Seattle, and take a flight from there (no requirement to test negative if your flight is a domestic flight). And, hope you don't develop any symptoms that would require you to be tested for covid.