Why was "wani" (crocodile or alligator) chosen as a term for a gawker at a konyoku (mixed gender onsen)?
Was it because of the animal's ability to stare, the fact that it is a predator, or was it named after a similarly named onsen?
I think that modern common sense would assume that men at kon'yoku who wait for women to come would be similar to the crocodile who quietly waits for their prey. However, while easily understandable, think that is folk etymology.
The term is used outside of hot springs as well. Also, you need to remember that crocodiles originally did not originally habitat Japan. The term wani originally referred to another creature, generally thought to be a type of shark.
Early citations for this usage may be found from the early 18th century. It seems that it is an analogy to these ferocious shark predators.
Crocodiles are ambush predators; they hide mostly inside water and wait for prey to come within striking distance as a means of predation. By analogy to this, men who wait for women in mixed-bathing hot springs are called ワニ.