The airplane itself, a Grumman Duck, that was used in the movie and flown by Frank Tallman, can now be seen on static display at US Air Force Museum in Dayton, Ohio.
The part of the U-Boat was played by a Venezuelan submarine, the former USS Tilefish (SS-307). The floating crane was, itself, a former WW2 tank landing craft.
Apparently, despite a copyright credits statement made on this 1971 film, this movie wasn't actually registered for copyright until 30 January 1990 (copyright number PA-490-129).