The answer is:
The poet uses repetition to highlight how much water surrounds the sailors.
Repetition is a literary and rhetorical device which involves the recurrence of a word or phrase for emphasis, to add intensity and to make the speaker's ideas and thoughts more straightforward.
In the passage from "The Rime of the Ancient Mariner," the author Samuel Taylor Coleridge makes use of repetition to make more forceful the fact that the sailor is thirsty in a motionless ship, in the middle of water but unable to consume it.
A comma is used to show a short pause in a sentence
