Moisture control is the big issue. One head usually becomes a shower room, which also will do double-duty as the drip locker for foul wx gear.
You'll want to remake that head so it is a "smooth" as possible, fewer corners and textures and nooks to hold water. If the trim is wood, reline it with Formica or other plastic trim and seal the corners with appropriate trim, using "bathroom" grade sealants which have arsenic or other anti-mildew agents in them. Cracks & corners are your enemy, anything that gives mildew a chance to establish a foothold will.
Install a solar powered vent fan in that head, so that there is always air being EXTRACTED out of it and out of the boat. That will make the biggest difference in keeping it dry and mildew-free. After a shower, don't towel down the walls (the towel has skin & oils, aka mildew food, on it) but use a rubber squeegee to wipe down the surfaces. And finally, a bottle of after-shower-spray (any brand) which helps ensure the moisture runs down quickly and has some anti-mildew properties as well. That stuff REALLY makes a difference.
If you can also install a louver or wicker vent in the lower section of the head door, in a way that the shower won't spray OUT, that also helps the ventilation. If not, get a door latch that allows the door to be latched "ajar" so it remains cracked open without banging around.