At the time where they are baiting the croc, Hector announces that he's getting something on radar, but then announces that the croc is underwater. DUH, radar is for the air, and sonar works underwater, and you can't go from one to the other. Someone on the ground should have been the one to announce this.
MAYBE he was above the water when they found him on radar, and then he disappeared, so he assumed that the croc went under water, because it wasnt likely that a croc would just disappear without a trace, when theres a lake that he could have gone into, where the radar wouldnt work.