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?
In the first instance, the depth of the water is the depth of the specific medium below the boat. A boat also floats on water, but this would not refer to the physical body of water on which the boat sits but rather the generic medium that keeps the boat afloat. If the boat floats on the water, we better know which exact river, lake or sea the writer is talking about.
In the second instance, the use of the verb (to fathom) is figurative, and the imagined measurement is not of a specific body of water but rather of the generic medium 'water'.
Similarly, while we drink water to quench our thirst (the generic liquid), when there's a pitcher of water (generic) sitting on a table but out of reach, we ask our neighbour to "please pass the water" (the specific thing).
the depth of the water - 'the' is used because 'water' is identified/known or is identified in the mind of the speaker/writer.
the depth of water - no 'the' as the speaker/writer is thinking of water in a general sense.
A similar example would be 'Sugar is usually white' (meaning 'sugar' in a general sense).
In the first instance, the body of water is identified (one is over it on a boat) and a line is let down into that identifiable volume of water. Therefore it is identified with the definite article.
But on land, one is doing what is termed by the article 'the figurative equivalent of measuring' the depth of water. In that case it is a calculation and the water is just water as such, in concept. Therefore it is not identifiable and does not warrant the definite article.
