Final answer:
Rainsford's sudden external conflict is with a treacherous storm at sea, which represents a classic struggle against the forces of nature in literature.
Step-by-step explanation:
The external conflict Rainsford is suddenly confronted with is a treacherous storm at sea. This conflict is one that pits him against the forces of nature, which is a common theme in literature where characters must survive against natural calamities. The details given, such as the ship not arriving at its destination, wreckage being seen by other sailors, and the implied reasoning by Dr. Watson, suggest that the only reasonable explanation is that the ship was destroyed by the storm. This signifies the struggle that humans often face when confronted with the unpredictability and might of natural events.