I had the same dilema. We just returned from the cruise, and we did the Nassau Dolphin/Blue Lagoon beach day last Monday. I had purchased an expensive digital camcorder with still photo capability befor the trip, and felt the justification would be this day. I signed my two kids (DD 8, DS 6) for the 'encounter' vs. the 'swim' which was about $88 per kid (booked through Disney). I planned to be the designated photographer, and capture the Kodak moments. Weeks ago I began calling about adding myself to the package thinking there might be a way to do both (film and encounter) since the more I thought about it, the more I wanted to be a part of the action. The group is set by numbers, and I was told it was sold out, but I could try to add myself when I got there. Sure enough the green worked, at the same $88.
Before I paid, I was told that I could film from a platform about six feet over the 'area'. The area is a square floating dock with a 20' square opening. Everyone sits around with their legs in the water waiting for their groups (about 10 people at a time) turn to stand on a submerged platform and 'encounter'. Trouble is they have their backs to you (you being the cameraperson - six feet above them on a seperate dock). When I paid, they allowed me to film down around the square dock, and I got some great footage (at my own risk as the dolphins do some splashing). Some of the 'official' photographers are not so pleased that you are doing this, and a couple of times I was told to sit down with the others. Then I joined the last group for my encounter and had some people use my kids' waterproof (disposable) cameras to document my encounter. I might recommend trying this, but be prepared for some cold shouldering by the staff (unless you intend to buy their video and stills and not bother doing it yourself). Have fun.
