Wouldn't you call in a drunk driver? This person was putting lives at risk just the same as drunk driver, if not worse. She was actually getting paid to protect people!
Yes, you should have reported her, but you didn't. I always do the same thing, and later beat myself up over it. But this might be one instance where you might still be able to do something about it.
If you can tell them when and where you were and provide any sort of description on the lifegaurd, they should be able to figure it out. I would think they would take this very seriously considering their liability had anything actually happened!
All you can do is try. A lot of what goes wrong is this world today happens because we're lazy and take the easy way out. Yes, it would have been a pain to do something about it with the stroller and kids and all, but sometimes it's worth the extra work. You might just save a life.
And that is in no way intended as a judgement, more of a pep talk to myself for the next time I'm in a similar situation. If we all just did a little better, tried a little harder, spoke up a little more often, think how much better things would be!