It's not safe to walk into water, let alone swim, anywhere in Kakadu National Park--because people-eating saltwater crocs could be anywhere.  In light of that well publicized information, this sign reminds me of one I came across in 2004 down in Arizona's--or actually the Havasupai tribe's--Havasu Canyon, right at the top of 190-feet-high Mooney Falls (higher than Niagara):  "No jumping."

Anyone who needs that kind of warning is too stupid to get very far anyway.