"There may be freshwater crocodiles in most parts of the river, as they nest along the banks, but they are harmless to humans." - Wikipedia

This is during The Dry season but when it is The Wet season, saltwater crocodiles can enter parts of the gorge and they are removed by park rangers, to the upper river. Therfore, swimming during The Wet season is prohibited. I would NOT swim at all but enjoy the stunning landscapes from the safety of the cruise boat!!