I personally don't go swimming in inland rivers or billabongs...in fact, I don't go swimming anywhere I can't see the bottom - that does it for me. There's usually half a dead tree, branches intact, sitting on the bottom of the waterhole, and the difference in temperature between the top 3 feet and the rest is something phenomenal sometimes.
If my Koori mates tell me to chuck a rock, if I get close to water, which is to warn the Water spirits, then I reckon it's a good idea to not swim there...besides...Bunyips live in water, and they're worse than drop bears...