Legionella is a freshwater bacterium, so it's possible that your water is naturally too salty for optimum reproduction. It's hard to say. There are a lot of different factors in any case of disease transmission

There was even a study in the UK that pointed to the windshield wiper fluid tank in vehicles being the source of infections in the drivers, mostly people who drove for a living. In those cases, plain water was used, instead of wiper fluid. The bacteria grew in the tanks and became aerosolized whenever the drivers sprayed fluid onto their windshields.