I have found that unless there is nothing wrong when a bathroom is to be remodeled the best solution to fix any problem to to completely gut out every thing and start with a clean slate.
In this nest set of pictures you can see a small spot of rot on the floor between the shower and bath. This rot extended under the shower and up the dividing wall which drew ants into the home, even though it was on the second floor. There was a faint water spot on the ceiling in the room below this bath. Here the home owner chose to go with the schluter shower and tile system and hired me to guarantee it was done right.