I Grew up in Kununurra, and spent most of my childhood swimming with Crocs we swam in areas that were know to have populations of Freshwater crocks, but we also knew Saltys were capable of being in the same area. predominantly our swimming was kept to popular boating\swimming areas the high amount of activity tended to see the crocks stay away at least during the day.
there were warning signs up around known crock areas and no swimming signs up around known salty population. but as i said above there is nothing stopping the odd salty from visiting a freshwater area for some time, but its just part of the area you live in you have to be aware of the dangers and play it safe as you can but there will always be exceptions.
i distinctly remember 1 weekend swimming in a channel with my mates, the next weekend a kid was taken by a salty in that same channel, and that was\is one of the popular swimming spots for the local kids, its unfortunate but **** happens and as i said it just apart of living in that area and sharing the waterways with large predators