About a mile of Atlantic-facing sand, and the best swimming on Key Biscayne for anyone who is not managing a toddler.
Versus Crandon, ten minutes north: Crandon has a sandbar and a flat lagoon, which is better for small children and worse for everyone else. Bill Baggs faces the open ocean, so you get actual waves, deeper water sooner and noticeably better clarity.
Versus South Beach, across the bay: no crowd, no hotels behind the dune, no music, no rental-bed grid. The trade is that there are no shops either. Bring lunch or use the park concessions.
Practical notes. Lifeguards cover designated sections during posted hours, not the whole mile. The beach faces almost due east, so this is a sunrise beach — genuinely one of the better ones in the city, with the sun coming up out of the Atlantic and nothing between you and it. Shade comes from built shelters and the seagrape line rather than from trees on the sand.
Seaweed and sargassum land here in some seasons, mostly late spring through summer, and the park does not groom the full length of the beach. That is a conservation choice rather than neglect, but it is worth knowing before you arrive expecting a raked resort strip.