I checked the tide tables for Edgartown. Peak to peak tides in the month of July, 2009 are less than 3 ft. (1969 would have been very similar) High or low tide probably wouldn’t change the level of Pocha Pond, very much, obviously. It is, however, narrow so that during the change in tides there would have been a very strong currents making swimming or opening and closing doors in a submerged car difficult.
Interesting, to say the least. The problem with Kennedy is that he didn’t seek immediate help for her. For that he is a cowardly SOB.