It's surprising sometimes how much of a book you read forty years ago will come back to you once you pick it up and start reading it again. I own the hardcover, and I remember exactly where I bought it, a little thrift store in Austin. I was in Austin again last weekend, saw the paperback, and picked it up. You can never have too many books.
You can see the blurbs on the cover, but the best one is from Publisher's Weekly on the back cover: "A ripsnorting nautrical swashbuckler of the kind 'they don't write any more' . . . ." If it was the kind they don't write anymore in 1969, that must be a lot more true now.
Simon Algive is an underwater demolition expert who's recruited by an old friend to recover some diamonds from a sunken ship filled with bombs. The diamonds are hidden in the bombs, which have to be disarmed. You already know that things don't go as planned. They never do. There are double crosses. Lots of people die, both above and below the water. There are those bombs. And romance. But here's the kicker, something that I'm sure James Reasoner will agree improves any thriller: quicksand. You can't go wrong with quicksand. It takes a while to get things sorted out in the end because things are much, much more complicated than they'd seemed. All of this is smoothly told in Algive's first-person narration. I was reminded much more of Alistair MacLean than of Ian Fleming (see blurb on cover). Fine stuff.
Dipper went on to write five more novels, which I haven't read. I don't know why. I should have sought them out.