As an Aussie what the person below has said is a big one here. We just call them rips. Basically if you just try to swim in them normally you won't go anywhere and will just make yourself tired. Same goes if you're caught in a rip and trying to get out. It can lead to people drowning from tiring out and going under. What you want to do is swim diagonally across the rip. Then you can go about your swim or swim safely back to shore. Another tip is if you don't know what a rip looks like then it can be hard to see them from the shore or while your in the water. They aren't waves.

https://www.google.com/search?q=beach+riptide&tbm=isch&client=firefox-b-m&hl=en-GB&sa=X&ved=2ahUKEwio2KnNkI6BAxWEamwGHV0UAmwQrNwCKAB6BQgBEK4B&biw=678&bih=708

Another one I think people usually have issues with or you hear of a tourist going missing is swimming in water inland. This is more of an up north Aus thing. Basically if you can't see into the water your going to swim in them don't. Crocs like to hang out in that sort of water. Very easy to not see them at all.

He must be referring to riptides. In some spots the water hits the beach as waves. In others nearby, the water gets pulled back into the ocean, and those are the spots you need to avoid.
Then depending on the ebb and flow of the twice-daily tides, the riptides are stronger or weaker.

There are ways to see where the riptides are, yet many people from my own coastal town are oblivious to these dangers. Inland/landlocked tourists are even more oblivious and vulnerable.