A citation from the movie The Town:
And one of the guys told me that… to walk until I felt the water on
my toes. It was the longest walk in my life. I kept thinking I'd step
off a cliff. And then I felt the water.
Why is it the water and not just water? Any water feel alike. They could have gone with just water. There is no possibility that you feel some water on your toes and say: "No, it is not that water I was told about, I should keep going until I feel the water."
Best Answer
The definite article is there to emphasize which water they mean. The robbers/kidnappers want to make it clear they mean the water of the ocean, not just any water. Otherwise the kidnapped man could immediately spit on his toes, then take off his blindfold and recognize his kidnappers :)
Even though they leave "which water" unspoken, the implication is clear to the kidnapped man because they are on the beach.
Realistically, the first "the" could have been left off and it still would have been clear. The second "the" I would keep, because it poses a question: which water? Answer: The water on my toes, the water that ended my ordeal. It's more poetic this way :)