Albert Barnes 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)

And sounded. To sound is to make use of a line and lead to ascertain the depth of water.

Twenty fathoms. A fathom is six feet, or the distance from the extremity of the middle finger on one hand to the extremity of the other, when the arms are extended. The depth, therefore, was about one hundred and twenty feet.

Fifteen fathoms. They knew, therefore, that they were drawing near to shore.