Final answer:
The sentence is grammatically correct but logically unsound.
Step-by-step explanation:
The sentence 'The curling crest in a plunging breaker occurs because they are the best waves for surfing' is grammatically correct but logically unsound.
The sentence suggests that the curling crest in a plunging breaker occurs because they are the best waves for surfing. However, this is not a logical explanation. The occurrence of a curling crest in a plunging breaker is a natural phenomenon, and it is not dependent on the quality of the waves for surfing.