Charles Ellicott Commentary Acts 27:28

"and they sounded, and found twenty fathoms; and after a little space, they sounded again, and found fifteen fathoms." — Acts 27:28 (ASV)

Twenty fathoms.—The Greek noun so rendered was defined as the length of the outstretched arms from hand to hand, including the chest. It was considered as equal to four cubits—i.e., to about six feet—and is therefore fairly represented by our “fathom.” The soundings here given agree with those that have actually been taken among the breakers off Cape Koura.