Answer: Probably to show how time-consuming and frustrating the search was becoming. Possibly to show that their systematic and painstaking efforts were likely to produce the desired result: finding the wreck if the Titanic.
You have to read the selection to know for sure.
Explanation:
The answer choices given don't always match up with the reader's perception and understanding of the selection. Perhaps the class discussion or lecture would have provided a clue about what the teacher expected students to notice.