Full of Canarians at weekends and tourists on Mondays and Fridays because of the markets, it's best to visit Mogán beach midweek. You'll get a better spot on the sand and a better table for lunch.
Because it's sheltered by the harbour wall, the water at Mogán beach is almost always warm and calm. It's perfect for swimming and for families. The snorkelling is excellent if you head to the far end of the beach and jump in off the pebble beach (don't do this on rough days).