Answer: The sentences that include only prepositional phrases are “There was no one visible; under him, in the water, the dim shapes of the swimmers had disappeared” and “He could see nothing through the stinging salt water but the blank rock”.

Explanation: A prepositional phrase is a group of words introduced by a preposition. Generally, a prepositional phrase also includes a noun or a pronoun. The sentence “There was no one visible; under him, in the water, the dim shapes of the swimmers had disappeared” contains two prepositional phrases: “under him” and “in the water”, which have been introduced by “under” and “in” respectively. Furthermore, in the sentence “He could see nothing through the stinging salt water but the blank rock”, “through the stinging salt” is a prepositional phrase since it begins with the preposition “through”.