As far as I know the only thing these books have in common is that they feature diving and have a shark on the cover but I thought that's good enough for a Three of a Kind post!
Given the source of the contents, it's perhaps surprising that this was published under Pyramid's Willow imprint which was aimed at a juvenile readership. With stories such as Sharkbait Swimmer, Human Torpedoes and Dive or Die they must have been eager to devour this book! I got my information from the excellent Glorious Trash blog and you can read Joe Kenney's full review here.
This 1962 Spanish publication, which translates as Conquests and Mysteries of the Sea, was an album for a set of 128 collectible cards.
The Secret of Shark Reef, published in 1979, was the 30th book in the long-running Three Investigators series. The books were originally billed as "Alfred Hitchcock and the Three Investigators" and this was the last to feature the famous director before his death in 1980. This edition is actually from 1985 and the silhouette of Hitchcock that previously appeared in the top corner has been replaced by a keyhole.
The artist is Robert Adragna who painted covers for books 29 - 43 (the last in the series) as well as providing artwork for new editions of earlier books. You can read more about Adragna on this website devoted to the American editions of the books.