We cruised on Mariner on a post-New Years cruise in 2012. It was a 6 day itinerary with three ports of call: Costa Maya, Belize City and Cozumel (two days at sea). I'll break down my review into food & drink, entertainment, ports and logistics.
Overall grade: C
Food & drink
The dining room was very good on Mariner, with a high quality and variety of main courses, appetizers and desserts. The service in the main dining room was also superb. The general buffet restaurant was convenient and food quality was adequate, if not particularly tasty or interesting. The bars were diverse and fun. We liked the Viking Crown bar on deck 14 most, with music every night, a great view and comfortable seating.
The shows we saw on board were mediocre, including a second-rate Motown cover band and a set of singers in the feature show who were, for the most part, uninspiring and lacking any great talent. The ship's orchestra, on the other hand, we're strong and made up for some of the other weaknesses. The bands on the pool deck and the Promenade were of mixed quality but usually lively and providing good variety of music.
Costa Maya was an almost complete waste of time. We decided not to book an excursion here and that perhaps was our key mistake since the port itself was synthetic and boring. The obligatory Diamonds International store was there of course but otherwise the shopping was tent after tent of the same cheesy faux-Mayan junk. The "beach" was manufactured and didn't even reach the water, leaving no opportunity to swim or snorkel. Belize City was better but was similarly junky and synthetic in the closed port area. Outside the secured port area,the city itself was sketchy - poor and decrepit almost everywhere. We took an excursion to a zip lining place. It was over-priced at $99 and required a jerky, hot 40min bus ride in both directions. We loved Cozumel. We did the Amazing Cozumel Race and had a blast. It left us with a great desire to return and perhaps stay for a week next time.
Unfortunately, handling of embarkation and especially disembarkation is where this cruise failed utterly. Starting our cruise experience with three long lines in a hot terminal put us on the wrong foot right away. But it was the disembarkation that was truly horrible. We had a fog delay which put a lot of pressure on the process to be sure but, frankly, I saw no real respect from RCI for our time or comfort as we left the ship, claimed our bags and made our way snail-like hough the extremely long and confused customs queues. It seemed to us that they washed their hands of us assoon as e stepped off the ship - caring far more to bring on new paying customers than to provide a smooth and stress-free cruise completion experience. I think the US Customs staff deserve some, perhaps even most the blame for our painful experience so, to that extent, this criticism is as much about the Port of Galveston as it is about the indifference and incompetence of RCI ground staff. Despite an otherwise pleasant cruise, we've sworn off both RCI and this Port because our experience (5+ hours to disembark) was so torturous.