In the context of the historical setting described in the passage, the term "alligator" refers to a low armored vessel, specifically the LVT (Landing Vehicle Tracked). This type of vehicle was used in amphibious assaults during World War II to transport troops and equipment from ships to the shore. The passage describes the sounds and sensations of being in this vehicle as it makes its way over the reef, indicating its function as a means of landing on the beaches during the invasion. Therefore, the correct response is:
a low armored vessel.