Final answer:
The author creates a hopeless effect by comparing Gerard's inability to stop the undertow to that of a wooden puppet, highlighting his helplessness in the situation.
Step-by-step explanation:
The effect created by the author in the sentence: "The undertow was like a string pulling him, and he was as useless as a wooden puppet to stop it" is a hopeless effect, making the reader understand the desperation Gerard must have felt at that moment in the water. The simile used compares Gerard's inability to stop the undertow to that of a wooden puppet, emphasizing his complete helplessness in the situation. This creates a feeling of hopelessness and highlights Gerard's lack of control over the situation.