Though snakes that live (or just hang out) in water are not dangerous, the really bad snakes are the ones that live in deserts. (I think I learned this from NatGeo)
I'm not sure that you should rely on that as a rule.
Some sea snakes (at least in Australia) are venomous. Also, snakes (including venomous ones that might normally live out of the water) can swim and in flooded regions would be forced to do just that. The red bellied black snake isn't as dangerous as the inland taipan or eastern brown, but it does like water.
Some of the most dangerous snakes in Australia always live in and near watercourses, and are usually disturbed while sunning themselves near a river when someone decides to go for a swim. The types of snakes found swimming around in the Queensland floods are not the types you want to be messing with.
