My heart goes out to you - I know you're a great owner and this must be so frustrating, especially out of the blue like this.
I'm clueless too, so probably wont be of much help but if he is not only guarding the water bowl, but also drinking excessively, that could be a sign of something more medical related rather than just resource guarding on it's own.
But if he's checked out fine at the Vets (no diabetes - UTI ect) could it be that he likes to mark and perhaps with him being in a new place, he feels the need to "mark" more often? Drinking more = ability to Mark more area?? I don't know, grasping at straws here.
Have you considered just giving them ice cubes as their water source for a few days to see if that makes a difference or Maybe give them each a water bowl on opposite sides of the room???
I wish you all the best and hope you can figure something out.