This full-length Saint adventure, although published in 1941, appears to be set just before the outbreak of war in Europe; it seems unlikely that Simon, Pat and Peter would be idling about on a Florida beach otherwise, even in response to an appeal for help.
This is thus the last of the pre-war Saint stories, the last where the Saint is still clearly British and not a naturalised American, and, perhaps not coincidentally, the last before the quality starts dropping through the floor and the plots (with a few exceptions) become formulaic.
Nonetheless, Nazis and pro-Nazis are the villains of this one, which also features our old friend Hoppy Uniatz, in fine form. The incidental details of Florida, the Everglades and their denizens seem to my uninformed judgment totally convincing, and a friend of mine considers this possibly the best Saint story.
P.S. For a list of all Charteris's Saint books (in two sections, because of length limitations) see my Listmanias.