I just got back from New Orleans, where I visited the National WWII museum (formerly the D-Day museum). Very interesting and moving. Here are a couple of relevant pictures:

Ephemera from the Higgins Boat company. Andrew Higgins, a native of Louisiana, had designed a small boat for operating in shallow waters; he modified the design into the familiar drop-ramp landing craft we've seen in all the films.

Models of Higgins' original boat (top) and the support and landing craft versions.

Engineering drawings

Not sure if the pics will show this way, if not, the links should work.