If that was my snake, I'd call the vet's office now, ahead of the appointment, and ASK if they can & will do a culture this time. If you wait until you're there for the
appointment, you'll owe them for it regardless of whether or not it's helpful.
I doubt the issue is "inhaled water"...I've had various snakes grab (f/t) prey & tumble into their water bowls to finish eating...never had a problem. It's far more likely that he came with the infection from previous exposure & it just took a while before it was obvious to you. The immune system of a snake is affected by stress (just like ours, presumably) so being in a new home didn't help...it just took time for the infection to be more visible.