The presence of a Crocodile under water may be indicated by small air bubbles rising to the surface. Do not go near any body of water which may contain Crocodiles if you can help it.
Crocodiles are notorious for killing humans, usually attacking people wading in the shallows. If attacked and you don’t have a firearm, your only hope is to stab it in the eyes with a knife, or sharp object, or even your fingers.