I think that Covid has made so many things that cruising is well known for obsolete that it is going to take the industry many moons to figure out how to redeploy ships in the Covid era.

Just think....
How do you do buffets? This is or was an integral part of most "mega" cruises because there wasn't enough seating in the dining rooms for all the customers at meal times. I currently see no safe way to do a buffet line anywhere (cruises or Las Vegas )
How do you do entertainment? Singing is a great way to spread the virus which is why Broadway and other vocal venues are currently shuttered.
How do you protect your crew? While you may be able to separate passengers by limiting numbers (thereby making the cruise either unprofitable to the industry or unaffordable to the passenger), you have to service your guests, and cruise ships packed the staff into cramped and close knit quarters. How will they be able to social distance?
Finally, what do you do if one of your guests comes down with Covid. Unless and until countries and ports have a unified and rational approach to how cruise ships will be permitted to dock and off-load such passengers, there will be uncertainty as to whether one should even step onto a cruise.

Advertising takes next to nothing to develop for a cruise. Pictures of pretty people enjoying lavish surroundings in in beautiful settings takes little to generate and almost nothing to print and send.

Unfortunately, cruise lines have to deliver what they advertise (in some fashion ) Until they can figure out the above, why promote what they cannot deliver