This ferry already exists. It operates back and forth from New London Conn. to Orient Point Long Island. Not far from the Hamptons.
The Cross Sound Ferry boat the Cape Henlopen(see below) was an LST at D Day on June 6, 1944. See picture below of the plaque that is on the boat.
And read all about it here:
The Cape Henlopen Ferry was built as a World War II landing craft (LST 510) in Jeffersonville, Indiana and participated in the D-day invasion at Normandy. In 1966, it was converted to a passenger and auto ferry and served on the Lewes, Delaware - Cape May, New Jersey Route. It was purchased in 1983 by Cross Sound Ferry and underwent a total refurbishment before entering service. The ferry was repowered during the winter of 1995 with EMD 12-645 diesel engines, ensuring a long future of trouble free operation.
http://www.youtube.com/watch?v=OAwXEDrevT8
https://www.longislandferry.com/Common/Help.aspx?page=fleet
http://www.newyorker.com/online/blogs/culture/2013/07/cover-story-operation-neptune.html#slide_ss_0=1
