I agree that the mercs were probably aware that there were dinosaurs on the island ahead of time, but how exactly do you train someone for that? Especially because I doubt they would have been informed of this prior to the power shutting down and actually getting the mission to go in. As was said above, the Park had it's own security force. I would assume that they knew at best a few hours before we see them in the game that they're going to the island in the first place, let alone what's on it. And it's not like the average mercenary force is going to happen to have weapons laying around capable of easily putting down a T-Rex.
Totally agree about the jumping into the water thing though. It was such an obvious thing to do.