Are you having a cystoscopy or some other scan?
https://www.mayoclinic.org/tests-procedures/cystoscopy/about/pac-20393694
I've had three cystoscopies. First numbing gel is injected into your urethra. After it takes effect, the 'scope is inserted (going around the corner at the base of the penis is slightly uncomfortable). Then the bladder is filled with saline solution so that the interior bladder walls can be visually examined. Perhaps drinking the water beforehand helps to flush out clots etc that may remain in your bladder.