Interpreting apparent competative advantage based on relative occurances of species in the fossil record is a very tricky business. For get humans for a start. Carnivores such as various cats and monitor lizards have been seen to be taken by crocs. It's probably why crows aren't favorite prey items. While it is possible that hadrosaurs, or any other dinosaurs for that matter, may have had warning systems to alert others of danger and that these systems could have been used to warn of crocodiles, it will remain unknown exactly how effective such systems were.
Modern animals eaten by crocs also have communication systems for danger but still they get eaten.
Do not underestimate the stealth of a croc on the hunt. There are numerous documentaries that show zebras and wildebeast quietly drinking at a river only to find that there is a croc lying in wait in the middle of them. If modern mammals have trouble seeing a hidden croc, I fail to see how a dinosaur would have faired any better. The latter would be particularly helpful in allowing an animal to see under the surface of the water because the polarization of reflected light would be different from the polarization of light transmitted from under the water's surface.