I read this book about 1-Apr-2012. I've read this book before. The book is copyright 1956. This note was last modified Saturday, 03-May-2014 20:50:53 PDT.
This is book 11 of the "Rick Brant" series.
This note contains spoilers for the book.
Published as by "John Blaine", a house name. As it says in the books, this is "A RICK BRANT SCIENCE-ADVENTURE STORY".
I only managed to find one of these when I was in the target age range for them—The Eledctronic Mind Reader. As I remember it, I got that the summer we were in Richmond, so that would be 1968. I never saw them in bookstores in Minnesota (not that I had much access to bookstores in Minnesota; there were none in Northfield until about then).
The obvious comparison to these is the Tom Swift Jr. books. I read considerably more of those, and did not like them as much. Tom invented more science-fictional things, but Rick Brant was more believable, and more fun. The Brant books felt like Heinlein juveniles.
I retained fond enough memories of them that I borrowed some from collector friends to read. And now I find that a selection of them are on Project Gutenberg.
In this one, they fly down to a Caribbran island (Rick flies the two airplanes they have on Spindrift Island where they live) for a diving vacation. His friend Scotty and two scientists from the foundation are along (the plane they use is a Cessna Skywagon, a push-pull twin-engine plane that doesn't need twin-engine licensing; it was originally developed for the Marine Corps.). They are going to dive for the wreck of a pirate ship they think they have found a clue to the location of in old log books.
But they run into a friend who is some kind of spy, apparently there on business. And they run into mysterious foreign divers who try twice to scare them off (or worse). And discover that these people are diving in the same area, when they head off to the little island they want to dive near.
Considerable underwater adventure, and use of spear guns and knives. Not much actual carnage. Scotty is an ex-Marine, but they don't actually fight much. Rick does use an underwater infrared movie camera (with infrared illuminator) that he has developed himself, though. It appears to come from Smugglers_Reef.
They do find the wreck, and they even find the alleged gold figurine of St. whoever that they never really expected to locate. In fact, they find a dozen or so of them, which on examination turn out to be lead, only coated with gold. One was apparently blessed by somebody important, which lead to it being written about in such terms as to lead people to believe it was valuable. (Interestingly, that's about the only mention of religion I remember in any of these books. You'd think such a mainstream normal family of good guys would be religious in a 1950s book, but I don't remember any sign of it. And the blessing of the statue was essentially treated as superstition (though that could just be anti-Catholicism).)
The foreign divers get rolled up, too, and it seems that they were deploying or at least testing sonar devices to help locate submarines. This is a major cold war gambit, of course. (One of them was placed in a cave where an octopus lived, and from a distance the sound it made was attributed to the octopus, hence the title.)
These stand up very nicely as light phone reading for me.