"The burgee on that swimming platform does look remarkably like the burgee for Larchmont Yacht Club.These photos are from the Emerson collection of Rochester N.Y.. Other photos in the collection fix the time frame. There is no explanation why young Mr.Emerson was at Larchmont YC but he did, in 1917, marry the daughter of Fred A. Mabbett, the Commodore of Rochester Yacht Club. Perhaps Fred Mabbett had chartered a yacht on the East Coast.There are some yacht racing photos in this collection.
"And Manor Park in Larchmont is famous for its gazebos and striated rocks.
"If you look at Manor Park on Google Maps satellite view you can actually see two gazebos on striated rocks very much like those in the second picture. But I can't identify the exact spot with those specific striations.
"Could this be a more recent photo of the same spot?
"Note that the gazebo may have been rebuilt in 100 years and a stone wall added. But look at the pattern of striations in the rock.
"Check out this photo from the Larchmont Gazette.
"Not the same day but surely the same view? Look at the trees on the point in the distance, for example.
"This is a postcard from Horsehoe Harbor in Larchmont c.1900. There is a Horseshoe Harbor YC (which is probably why your correspondent from Larchmont YC said it wasn't LYC but he did know it?)
"Oh, and I forgot to mention, Horseshoe Harbor is in Manor Park.
"On the other hand check out this photo of Ladies Day at Larchmont YC in 1911.
"This looks like a better match to your scene than that postcard of Horseshoe Harbor. The trees on the point look similar in both pictures but this one is claerly a wider stretch of water more similar to your picture. Would also explain why the swim platform has an LYC burgee.
"Maybe your header photo was of Ladies Day at LYC?
"This NYT article from 1910 does say that the fleet was dressed for Ladies Day.
"And this NYT article from 1913 about Ladies Day at Larchmont that year says that more than 200 yachts dressed ship and that possibly the fleet was larger in "former years."
"So I am pretty sure both photos are looking across Larchmont Harbor. If you look on a map you will see that the views from the gazebo in Manor Park and from LYC are both towards the same point on the opposite side of the harbor, to the NE and E respectively.
"And there's a fair amount of circumstantial evidence that this could be Ladies Day. There were canoeing and swimming races on Ladies Day which explains a lot of the foreground in the header picture too.
Below are all the photos in the spectator set, including a larger version of the header photo.