Normally range around 7-8 feet. Extremes are around 9-10 feet. Hurricane Sandy (guestimating) would have been around 12 feet.

Oyster Bay has a reversing current (with tide changes) probably around a knot in each direction, depending on location and tidal conditions. I could easily believe that a chain without a swivel could twist and shorten enough, eventually, enough to shorten and lift the mooring.

I think that's the simple explanation. Everything else is BS. (The guy who placed the mooring wouldn't want me on the jury, if there was no swivel).