While I don't have any specific recommendations, the best beaches for that will be on the Gulf Coast. The water is clearer and much calmer than the Atlantic side (a lot of people describe the Gulf as being like a bathtub). I'd look at places like Clearwater, St. Petersburg and Longboat Key.
If you want a truly unique experience, I'd recommend snorkeling in a freshwater spring. Crystal clear water and lots of interesting things to see underwater. Look at Rock Springs at Kelly Park, Wekiva Springs and Blue Springs. These will be much closer to Orlando (some are just outside the city).
