Ashley24 - We did the dolphin swim back a couple years ago and loved it! Our group was my sister, her DS11, my DD9, and me. It is quite the thrill, especially when you realize you are in the water touching and playing with dolphins. The grand finale is when one or two dolphins push you across the water.
Bring your credit cards! There is a lot to buy if you want to. We bought the video and decided on buying a photo album. You need to decide beforehand if you want to buy the photo album (and I remember it was pretty pricey). The photographer makes sure to get pics of you and your group for each type of dolphin interaction and then arranges the pics in a photo album. I am so glad we decided to buy it. We look at it all the time.
We also bought the video. The thing with the video is it will include all members that were at the swim at that time of day (around 10 people). You can view it after the dolphin swim and then decide if you want to purchase it or not. It's fun to watch. You can hear some of the things people are saying in your group, so be careful what you say! Our video has a guy yelling that he was losing his suit when the dolphins were pushing him! However, there was no visual evidence of this on the video.
There is also a shop with t-shirts, postcards, etc.
Let me know if you have specific questions, and I'll try to answer them.