The Main impediment "IMHO" to a amphibious T-Rex is the carnivores it would have to compete with. The wetlands were not empty places just waiting to be exploited by a big predator. There were giant sized crocodilians that lived in those wetlands, crocodiles more than big enough to take down a T-Rex... in fact they probably were a big danger to a T-rex trying to take a drink from a waterway BITD (back in the day) You have to remember that Crocodiles are powerful predators, well adapted to their environment, exothermic, so their food intake was lower than a exothermic T-Rex. Unlike a T-Rex Crocodiles can go long periods without food and so can afford, metabolically, to sit and wait for food to come to them. A T-Rex with its bird like metabolism had to eat much more and to go out and hunt down large prey items on a regular basis. A T-Rex wes adapted to hunting down prey and was probably fast enough to do so with its huge muscular hind legs.
T-Rex was not well suited to semi aquatic living in its body plan, it's metabolism, or its behaviors and was as vulnerable as any other land creature to the true dominating predators of the wetlands... Crocodiles.