American Flight 44 was a scheduled coast-to-coast flight from San Francisco to Boston with an intermediate stop at Chicago. The aircraft left O’Hare Airport at 09:18. Approaching the Boston area, the first officer flying the aircraft from the right seat prepared to make the instrument approach and landing. After radar contact was established near the Boston VOR, the approach controller reported the crew to expect a precision approach to runway 04R. The weather included “partial obscuration, measured three-hundred overcast, visibility one mile in fog, runway visual range more than six thousand”. At 11:03 the crew switched frequencies to the PAR controller who talked them down. The airplane was a little fast on the approach and a little above the glide path. During the approach the runway visual range varied from more than 6,000 feet as reported to the crew, to as low as 2,200 feet. The fluctuations in visibility were caused by lines of fog which originated over the water area, then moved over the surface of the airport. The aircraft first touched down 3,165 feet beyond the PAR touchdown point, which is 3,417 feet down the runway. This left the plane with only 3,438 feet of runway remaining. the aircraft ran off the end of the runway, turning to the right and came to rest in Winthrop Bay at a point approximately 420 feet beyond the end of the runway. The aircraft came to a stop in approximately 16 feet of water, its landing gear resting on the bottom of the bay. PROBABLE CAUSE: “The captain’s decision to land in variable weather conditions precluding adequate orientation relative to location along the runway. A contributing factor was the failure to provide the flight with information concerning the deterioration of runway visual range values.”