You have been rather badly misinformed.
After digging, McGinnis found a layer of flagstones and thereafter, a layer of logs every 10 feet.
McGinnis abandons the digging at a depth of 30 feet.
In 1804, the Onslow Company arrived at the Money Pit and resumed digging, finding more layers of wooden logs, charcoal, putty and coconut fibre at 40, 50 and 60 feet. They kept on digging until they reached 98 feet, where they struck something that sounded like another wooden platform. By that time, the soil of the Pit had become waterlogged and they had started to bail out the water. Since the sun was setting, they decided to call it a day and knock it off. When they returned the next morning, they discovered that the Pit was flooded up to the 33 foot marking, and the water could not be bailed out.
Seems to me that there is pretty convincing evidence that someone made a large effort to hide something on Oak Island.