Southold
Early family-related history
In the Preston M Harrington III photo album, volume 2, some photos reveal the history leading to having a house in Southold. First are pictures from 1937 and 1938 in Westhampton. The Harringtons would have rented accomodations there. One group of these is from a group dated Aucust 28 - September 12, 1938.
Shortly after this trip, on September 21, 1938, a massive hurricane made landfall on Long Island, causing extensive damage to the Westhampton area and many other places. Wikipedia mentions that "In 1938, almost all summer homes on its barrier beach were obliterated by a hurricane resulting in twenty-nine local deaths."
The album includes beach pictures from 1939, including one of Prestion III standing on what appears to be a wrecked wooden pier. In 1940, the Harringtons started going to Southold in the summer instead of Westhampton. This was probably at least partly prompted by the hurricane-related damage at Westhampton. They may have been aware of the area beforehand. One photo from 1937 says both that it's of Westhampton and of Cedar Beach, which is hardly clear about it. This could be at Cedar Beach in Southold, or maybe there is (or was) a different Cedar Beach in Westhampton. (There is currently a Cedar Beach on the south shore of Long Island, but much closer to NYC and not near Westhampton).
The house at Southold had a copy of Everett Allen's "A Wind to Shake the World", which is about the hurricane.