More importantly, I strongly disagree that liability has nothing to do with it. If a lifeguard is liable for unintended harm in the course of his duties and he's in the area the company is contract to cover, the insurance company will pay to defend. If he causes harm OUTSIDE the contracted area, the insurance company will deny coverage.
This falls under good Samaritan laws, mostly. If a person, any person, breaks your ribs while giving you CPR that was needed, they have no liability as long as they did the CPR correctly. Of course, if they decided to do CPR by jumping up and down on your torso, that would be a different story. But if that were the case, the company's potential fault wouldn't change either way. The company's fault would be based on whether or not they provided adequate training.
Meanwhile, one thing that many people don't realize, is that lifeguards are legally recognized as professional rescuers on par with fire fighters, EMTs, and police. This creates a
duty to act on the lifeguard's part. Failure to act creates negligence. So, the greater liability comes into play if the lifeguard ignores the drowning victim. Despite the "swim at your own risk" signs, the duty to act is not absolved.
Here's comparable scenario: Imagine you have an in ground swimming pool in your back yard. You take all reasonable precautions. You put up a fence, you lock the gate, you even put up a few signs that warn against trespassing. And you even put up a picture sign of a mean looking dog in hopes of scaring away any young children who won't know how to read. But one day all this fails to be enough, and a five year old climbs over your fence and jumps in the pool. You walk outside and see the five year old drowning in four feet of water. You decide that "well, I put up the signs, no need to do anything, I'll just call 9-1-1. Do you think you're going to escape liability of some kind?