I'm not sure, though, if any of the RN Higgins MTB s were stationed at Vis. I don't know anything about MGB s but the RN also used some former US Higgins boats as gun boats (45th MGB Flotilla).
Arjan
I have to rectify my mistake here, the midget sub is actually a "Molch" (Salamander). Quite a few of these were also found in the Dutch town of Den Helder after the liberation in 1945.The midget sub was of the "Seehund" type