Approx. 6 feet.
But when the fourteenth night was come, as we were driven up and down in Adria, about midnight the shipmen deemed that they drew near to some country; And sounded, and found it twenty FATHOMs: and when they had gone a little further, they sounded again, and found it fifteen FATHOMS. ( Acts 27:27-28 )
Bibliography Information"Entry for 'Fathom'". A King James Dictionary.