Final answer:
The restrictive participial phrase 'swimming vigorously with the current' completes the sentence about the campers, indicating that those who were actively swimming were safer. These phrases are necessary to understand which subjects are being referred to.
Step-by-step explanation:
The campers swimming vigorously with the current were safer during pool time. This participial phrase indicates that the campers who were actively swimming with the current had a reduced risk during pool activities. Participles are verb forms used as adjectives, and in this case, the present participle 'swimming' is used to describe the campers, specifically referring to their action in the water that contributed to their safety.
A restrictive participial phrase is one that specifically limits or defines the subject it modifies and is necessary to the meaning of the sentence because it tells us which campers were safer. Without this phrase, we would not know which campers the sentence is referring to. This contrasts with non-restrictive participial phrases, which merely add extra information that could be omitted without changing the fundamental meaning of the sentence.