I would think that would be rare - since they are full boat charters - the sailing would show sold out on the cruiseline itself and any booking would have to go through Atlantis / RSVP. Atlantis (and RSVP) generally charter the ship for one sailing. We've been on 3 (2 RSVP and 1 now defunct TravelPride) riverboat cruises in Europe (which is a lot different from these large "party" type cruises). Usually the fare that RSVP (and presumably Atlantis) charges are at a premium (small) over the usual fare from the cruiseline directly (and some otherwise included items - such as shore excursions on the river cruises are charged extra). Destinations are basically the same (not sure if Atlantis/RSVP actually even has much of a say in it). We'd only do these charters for the river cruises in Europe since the demographics on the "regular" cruises are really quite old (think 70s average).