Hey, guys, sorry to hear about your dilemma. Hope the dog continues to improve and that you found someone to take your spot.
This is a scenario that pet lovers need to keep in mind. Even if you guys had bought the Travelex insurance that Atlantis was pushing, it would not have helped. No third-party insurance company considers a pet's illness to be a "covered event" that will earn you a refund.
There IS another option on most cruises (though it wouldn't have worked with a gay charter like Atlantis or RSVP): Take the insurance offered by the cruise line itself, rather than a third-party carrier. The cruise lines' insurance will usually allow you to cancel for ANY reason and still get a credit of either 75% or 90% toward a future cruise. In this case it would have saved you at least $3,000! (But again, cruise line insurance doesn't apply when a company like Atlantis charters the entire ship, unfortunately)