We did this excursion a few months ago. If you are signed up for the encounter or the swimming excursion (i.e. you're going to be in the water with the dolphins), then no, we were told that you can't bring a camera in the water with you. But one person in your group can sign up as an observer (and pay a little less) and they can hang out on the docks right by your area and take all the pictures they want.
If that's not an option, you've probably already heard that the prices for the pictures taken by Dolphin Discovery are outrageous, but remember that everything is negotiable. The posted prices are incredibly outrageous but we were able to negotiate them down to moderately outrageous ($100 for a CD of all 40-50 pictures they took of our kids and the dolphins).