It is true. Parts of the straits were 4000 feet before, and parts are now only 100 feet deep. Trouble is, not the same parts. The parts that were 4000 feet are still 4000 feet, and the parts that were 100 feet are still 100 feet. Some of the straits are also 1 foot deep. The part just by the beaches.
Maybe the buoys & other markers moved, and are no longer reliable so that what a chart says is 400', relying on the buoys, is really 35'.