Drywall - Repair

Bathroom ceiling has water damage from a roof leak that has soaked through and removed a portion of the drywall over the bathtub next to a vent. I would like to fix the ceiling so that the bathroom can be comfortably used without worrying about debris dropping atop a bather. Also interested in putting in a drop in bathtub to cover the window in the bathroom but this may be beyond the scope of this more pressing concern.