A man in his early 20s who swam across the mouth of Drakes Estero with two others during a family outing on Labor Day vanished during the return swim, which coincided with an incoming tide, said Point Reyes National Seashore spokesman John Dell’Osso. The park, a water team, a dive team and two helicopters have all aided in the search for the missing swimmer, whose identity has not been released. (The fog, however, prevented the helicopters from flying on Tuesday.) The mouths of bays are treacherous areas, Mr. Dell’Osso said on Tuesday: “It funnels. It’s really no different than a sneaker wave, the force of the water coming in or going out.” Mr. Dell’Osso added that the park tries to warn people about dangerous beaches, using social media and its website, but he hopes to get various county and state agencies together to brainstorm better ways to warn recreators of the dangers of the coast. “It’s horribly sad and potentially tragic, and I’m struggling with what’s a better or different means of communication,” he said.