There were plenty sightings of crocs and hippo's there in the past. There are signs up at the gate warning folks about them but from my personal experience i have never seen any crocs or hippo's.
I have been told some stories by peeps that they seen crocs there.
The nature reserve gaurds drive up and down there so you will be in trouble if they had to find you on a floattube in any case.
I am not a huge fan of RDW due to overfishing, it's 2 busy for me there and all the city folks storm there weekend after weekend.
It is however a beautifull dam and natural atmosphere is a keen factor there.