If I understand correctly. a red flag at a beach means dangerous conditions, strong surf or currents. It looks to me like the risk is terminal boredom at the resort at the end of Playa Langosta. I don't know if that is technically a river or estuary in the mid-ground but, with the tide going out, the flow at its mouth was swift. As best I can tell the place is popular with Central and South Americans, particularly Brazilians. Nobody seems excited by their visit.
