The sentence below comes from Word Smart II: How to Build a More Educated Vocabulary.
Fathoming, at sea, is measuring the depth of the water, usually by dropping a weighted line over the side of a boat. On land, to fathom is to do the rough figurative equivalent of measuring the depth of water.
Why the 1st sentence says 'the depth of the water' in contrast to 'the depth of water' in the last. Is there no difference in any way or is there any implication I don't know about?