"Tom Swift and the Electronic Hydrolung" (The New Adventures of Tom Swift, Jr., Book 18) by Victor Appleton II

The legendary Sargasso Sea and Devil's Triangle disclose new mysteries as Tom Swift searches for a pirated space probe bearing a priceless payload--cometary matter from the dawn of our solar system. The quest takes an eerie turn when Tom's new detection device reveals an inexplicable gravitational anomaly that may be a fantastic seacraft a half-mile wide! To pursue the undersea search in the jungles of Sargasso weed, the young inventor must perfect his electronic hydrolung and jet-thrust diversuit, which provide deep divers with the freedom and mobility of true marine animals. Yet looming over the project is a desperate plea for Tom's help in preventing a political assassination on the island of Mer-Soleil, a nation in upheaval. No sooner have Tom and his pal Bud braved the acid-oozing vegetation of the sea floor than an astonishing secret is brought to light-- a clue to a cosmic event of unimaginable violence that not only changed the face of our world but of human history as well.