have not been to many different beaches, but the one that sticks out in my mind as the most beautiful one is Clearwater Beach in Florida. We went there as a family in 2001 during our Make-A-Wish trip to Disney. We found a very calm spot, where the water was so still, very shallow and the sand was so white and beautiful. The kids were able to walk out quite far and even despite it being a cool day in February, it was a wonderful experience for them.
These pictures are quite dear to me too, because though we were going for our daughter (below) who has now had three open heart surgeries, little did we know our son Matthew (above, tallest one in picture) would be leaving for heaven less than four years later.