A Tomales Bay boater died off Dillon Beach on Sunday morning after his leg became entangled in a rope tied to a crab pot that had been thrown into the water. The boat operator told officials that as the pot was thrown into the water and sank, the man, 40-year-old Santa Rosa resident Thong Nguyen, was pulled down with it. Mr. Nguyen, who was not wearing a life jacket, died before officials arrived on the scene, according to Mike Giannini, the Marin County Fire Department’s emergency services battalion chief. The National Park Service is investigating the incident, and the small vessel, which Point Reyes National Seashore spokesman John Dell’Osso said did not appear to be a commercial boat, had three people on board that day. Mr. Dell’Osso did not know how much the crab pot weighed, but said the treacherous waters in that part of the bay, where currents can be at odds with one another, could have contributed to the death. As to whether a life vest could have made a difference, he said it was possible.