I think this can go a few different ways. I'm [pretty] sure Six Flags wouldn't do anything to jeopardize the safety of their employees or guests. So it's always possible that the lifeguard didn't take proper precautions. Maybe didn't have anything to drink with him, didn't take breaks enough, didn't say anything when he started to not feel well. It could be a lot of things.
With that said, it's possible the park was stretching the time the lifeguards should be out to safe money. Who knows.
Also, Steve, I think lifeguards at the beach go for a swim every so often when they trade off.