Secret sea

"A dying Cuban, terrified by a nameless, sinister 'tall one,' leaves naval officer Pete Martin a clue to a sunken Spanish treasure ship. With only a brash waterfront urchin as crew, Pete sets sail in his old schooner, pursued by The Tall One in a radar-equipped yacht. Outwitting the enemy in a storm, they find the prize guarded by a monster octopus, and the menacing black yacht again on their tail." -- from Scholastic Book Services paperback edition, 1968