Just south of San Miguel is Playa Corona. There's not really much of a beach scene, as the small stretch of fine white sand, perfect for a volleyball game, ends almost at the waterline - it is a snorkeler's paradise.
Most visitors spend the day at Playa Corona Beach Club, with plenty of palapa-topped rancheros, strung with hammocks, tasty snacks, and cold beer. You can rent snorkel equipment nearby, then just step right out into the water - the reef starts right offshore. Strong swimmers will be able to paddle out to much more famous (and expensive) Chankanaab Park, about 100 meters (328 feet) offshore.
About 10 kilometers (6 miles) south of San Miguel, Playa Corona is not currently accessible by public buses, although Spanish speakers could check to see if that's changed. Rental vehicles are always an option, but taxis ply this stretch of Avenida Rafael Melgar regularly; you probably won't have to wait long for a ride back to your hotel.
Although Playa Corona Beach Club is free, you'll probably want to indulge in a cold drink or hot meal while you're here. All those hammocks swaying in the breeze make this a tempting spot for wiling away the day.