The Disappearing Floor by Franklin W. Dixon

Frank?” Joe said.
    â€œNothing—the puzzle’s getting more complicated all the time. A college prof rents the place, a notorious jewel thief moves in with him, and now we have to fit Makin in somewhere!”
    The boys decided to check into Darrow’s background. But first they drove to the repair dock to see how work was progressing on the Sleuth. The manager promised to have the boat ready in three days.
    Frank and Joe spotted the Napoli moored nearby. They were hailed by Tony Prito, who suggested they all go for a brief swim.
    â€œHow about it?” Joe said, turning to his brother. “Our trunks are in the car.”
    â€œOkay with me!”
    Tony took the Napoli down the bay a short distance and they anchored at a pleasant spot in a sheltered cove. A cabin cruiser lay at anchor not far away. Frank and Tony took a quick plunge, then climbed back aboard to sun-bathe. Joe continued swimming by himself.
    Like a seal, Joe cut his way down through the cool, refreshing water. Then he swirled back toward the surface.
    Suddenly he felt himself seized from underneath. A brawny arm clamped itself around his neck in a choking grip and pulled him down!

CHAPTER VIII
    Rock Hounds’ Shadow
    JOE struggled desperately. He had already used up most of the air in his lungs even before he was attacked. Now he was being gripped beneath the surface, unable to call or signal for help!
    Joe kicked and threshed, but he could not free himself from his attacker’s iron grip. The only result was a tightening of pressure against his Adam’s apple and a vicious jab in the ribs. When Joe tried to squirm around to face his assailant, the man rolled with him.
    Joe’s lungs were soon near the bursting point. If only he could reach the surface!
    Aboard the Napoli, Frank began to worry. “What’s Joe doing down there?” he muttered.
    â€œMaybe he met a mermaid,” Tony quipped lazily.
    As Frank scanned the waters, he noticed an uprush of air bubbles about a hundred yards away. Wordlessly, he plunged over the side.
    Cutting his way downward, Frank peered intently through the wavering transparent greenness. His heart pounded at what he saw. Joe was helpless in the grip of a goggled frogman!
    As Frank stroked swiftly toward them, the frogman released his victim and swam off.
    â€œGood grief! I hope I’m not too late!” Frank thought frantically.
    Joe was limp, his head sagging as he slowly floated upward. Frank grabbed him under the arms and hauled him to the surface.
    â€œTony! Give us a hand!” Frank shouted as he broke water.
    Tony had been watching anxiously for a sight of the Hardy boys. At Frank’s call, he sent the Napoli gliding toward them. In a few moments they had Joe safely aboard.
    â€œGet back to the dock! Fast!” Frank exclaimed. He positioned Joe as best he could in the bottom of the boat and quickly began applying mouth-to-mouth resuscitation.
    The Napoli went planing over the water toward shore. The jouncing made Frank’s efforts doubly difficult, but by bracing himself against the side he managed to hold Joe fairly steady.
    Tony cut the engine and yelled for help as he brought the boat alongside the dock. Four men assisted the boys in lifting Joe out of the Napoli, then laid him full length on the dock. Frank quickly resumed mouth-to-mouth resuscitation.
    â€œI’ve called an ambulance!” someone shouted.
    But Joe was already reviving. Frank breathed a silent prayer of thanks.
    â€œBoy! You sure had a close call!” Tony said, squatting down beside Joe.
    â€œYou’re telling me.” Joe grinned weakly. “I must’ve swallowed half of Barmet Bay!”
    By the time the ambulance arrived, Joe was on his feet. He allowed the intern to examine him but refused to be taken to the hospital.
    â€œNothing doing. I’m okay,” he insisted.
    â€œWell, I can’t force you.” The intern grinned and turned to Frank.