I got married at Mandalay Bay in October and had originally planned to get married outside by the waterfall area, but decided during a planning trip that it was too loud with the waterfalls and traffic on the street.
I am not sure about the beach wedding but their pool area is usually pretty busy during the summer, so I'm not sure how they would keep people from watching or being near the ceremony.
I worked with Alyssa and she was very helpful through the whole process, I bet if you just call them they can get you more info on how they do the beach weddings. I believe Mandalay Bay was the best deal for the money that I found at any of the hotels.
Hope this helps!
Amy