You're right, it's not the 109 or even an Elco for that matter. Due to the wholesale destruction of PTs in the Phillipeans at the end of the war, few 80 foot Elcos were still in existance when the movie was filmed. To shoot the movie, 82 foot air force rescue boats were modified and used to represent the 109 and other PTs depicted in the movie. One of the few remaining, PT-617, is on display at Battleship Cove in Fall River Massachusettes. Pictures of it can be found at
http://www.williammaloney.com/Aviation/PTBoat/index.htm