What is the point of Les's swimming-pool lifeguard analogy?
The analogy shows that just as a trained lifeguard hears one feeble cry for help amid a noisy pool and springs into action, God hears a sinner's cry for salvation and immediately responds. A person must recognize their need, cry out, and God saves on the basis of faith in Christ.
Les explains that he prefers simple analogies anyone, even a child, can understand. In a swimming pool full of yelling kids, the crowd may not notice a weak call for help, but a lifeguard with a trained ear hears a child going under and springs into action. Les says it is the same with God: when God hears a sinner in need of salvation, He is immediately there to save. Yet the person must come to realize their need, cry out, and God responds. This pictures Romans 10: believers are to believe Christ died and rose, but God expects them to cry out on that basis for His saving power.
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.