Explanation:

- Original Statement: The sailors had never encountered such a rough sea.

- Option 1: Such a rough sea was never encountered by the sailors.

- Incorrect tense. It should align with the past perfect tense used in the original statement.

- Option 2: Such a rough sea had never been encountered by the sailors.

- Correct. It uses the past perfect tense, maintaining the meaning of the original statement.

- Option 3: Such a rough sea has never been encountered by the sailors.

- Uses present perfect tense, suggesting a different time frame that doesn’t match the original.

- Option 4: Such a rough sea is never encountered by the sailors.

- Uses present tense, not matching the past context of the original.