Me and my husband have cruised Marella with another couple, approx. half a dozen times. We don't pay for particular cabins, and we are all on the same one booking. we are put no more than a couple of cabins apart each time.
I took a 3 night Christmas cruise in 2014 on Marco Polo. Was not expecting a formal night. Horrors,,,,,,, there was one, and was unprepared. Managed to wing it, there were others who also were caught unaware but many were fully prepared , dinner jackets and bow ties. I would say, go prepared !!!!