The normal level of sand to bedrock on the atlantic ocean around cape cods east side out to about 2 miles?

How far could something sink in the oceans bottom in the sand b4 it hit bedrock. Like a shipwreck. So how deep would the sand get within two miles from shore around say the chatham mass area to nantucket. Just on the outside of the beach down to bedrock out to two miles offshore. Just adverage. I would think their are deeper holes,but just adverage.Thanks