I normally only book with tour guides that have been recommended by others. You have already done that. You can look through this forum for more names if you want. Reggie and Xtreme Cave tubing are the most used.
Also, make sure you will be refunded any deposit should the ship not port for some reason. All the reputable business will state this. Don't book with any guide that won't. Cruise itineraries get changed for many reasons (weather, mechanical troubles, etc) and the tour operator's in the business know this and won't hold it against you.
I'm not sure there is much more you can do. Unfortunately, there is not such thing as an international better business bureau or something you can check with. That is why the word of mouth thing is so useful.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007