Cutting and repairing drywall ceiling above shower stall
I've used a circular access panel made for boats (water tight) in a ceiling above a shower. Works very well, doesn't let moisture in. The link is a textured one for boat decks, but The smooth (non-textured) look best. 

Basically run clear caulk around the rim and in the screw holes. 
I framed some wood backing in there between the joists to mount the rim to. A few years on, all working well. I open it a few times a year as it's the access panel for a valve to an exterior faucet.