I think most people who own at BCV just love SAB. We stayed there for five days a couple of years ago, and we were frankly underwhelmed by SAB. My basic impression of SAB was that it is a wonderful place for adults to sunbathe, but not much of a place to swim.
Although SAB is large, it's chopped up into a bunch of very small sections. The only large area to use is the sandy beach area for toddlers. Everything else strikes me as small features that are interesting, but not that much fun. In fact, the second day we started to go to SAB, DD6 said she wanted to go to the pool with the sandcastle slide (OKW).
Other posters are right about deep water and strong currents. We have a pool at home and our DD6 is a very strong swimmer, so we weren't worried about her, but kids who aren't really strong swimmers will have to be very closely watched at all times -- and I mean being within a few feet of the kid. I was very impressed with the SAB lifeguards -- they are very attentive and have all the best EMS gear -- but I certainly would not depend on them to watch my child.
We were also less than pleased about swimming in the same water with ducks. Duckies are cute as they can be, but they are filthy birds.
SAB notwithstanding, we thought BCV was a spectacular resort. We loved the ambiance, the grounds, the staff, and the proximity to Epcot, the Boardwalk, and DHS. It's a beautiful place and we look forward to staying there again.