How do locals decide if the water at El Zonte beach is safe today?

I come from a country with a lot of coast, but I am new to this one, and the water at El Zonte beach looks different every morning. Some days people swim far out and other days nobody goes past the wet sand. The ocean currents are strong here, and I would like to learn to read the water instead of guessing. How do locals decide whether the water at El Zonte beach is safe on a given day?