Bull sharks spawn in rivers and other low salinity waters. They live in the ocean. As such, it's no surprise to occasionally find a large specimen in freshwater. However, to claim that the largest ones are in fresh water based on Jeremy Wade catching a big one in freshwater isn't a sound conclusion. The largest fish are caught in saltwater, as supported by
this fish caught in the Atlantic off of Kenya which is the IGFA world record, or
this one caught off of Miami that's even larger but uncertified. Take your pick.
The reason this reply took me so long is because even though Jeremy's bull shark was "the biggest ever caught", I can't find a weight listed anywhere. Odd.