If you've ever watched Psycho, you know that several, including the actress herself, have gained lifelong fears of being attacked in the shower from its most famous scene. I am one of them. While I certainly hope never to be in such a situation, I have cultivated a few self defense techniques should I ever be caught in an attack while in the shower.
Note: These were modeled off my shower, which has a curtain and rod. If you do not have these things, either modify the techniques requiring curtain and rod, use other techniques, or make up your own for the shower door. Also, please note that since this is based off the idea that a knife will attack you, I will not have any advice on how to defend yourself from a gun.
#1 Soap as an irritant
Lunge and aim for spots on the nose, mouth, and eyes. Even childrens' soap, designed to be gentle, will blind your opponent momentarily as they attempt to get it out of their eyes. And no one likes the taste of soap.
#2 Hot water
Turn up the heat, direct your showerhead, and spray. It might be wise to keep a cloth between the handle and your hand if you're touching the metal, as it will get VERY HOT! Yeah, I know, dumb warning, but you can never be too careful. Back in ye old Medieval Ages, pots of boiling oil were poured over walls to stop soldiers from climbing. This may be one of the best techniques as it requires nothing more than a shower and might be enough to subdue your opponent on its own. Plus, you'll generate a lot of steam, so if your opponent has something wrong with his eyes, this will bring it out
#3 Beat them with the shower rod
No need for further instruction here. If you've got a sturdy wooden one and some training, good for you, you're all set. If you have a flimsy metal one and no staff training, best of luck.
The curtain is one big piece of cloth. You can cover them with it if it's opaque, you can twist it into a whip, you can use it as a shield if it's thick enough. More importantly, you can turn it into a dress if fighting naked is too embarrassing.
#5 Use water as a slick
While this sounds directly out of James Bond, combination of a smooth surface and water is guaranteed to make even one of your opponents missteps a fall. Just get them off balance and they're going down.
#6 Plunger attack
Use one end as a baton, attach the other end over your opponent's mouth and nose.
#7 Throw whatever you have at them
Bottles, brushes, electric toothbrushes... whatever's heavy enough to dent.
#8 Smash anything that can cut
Glass, ceramic, your mirror. Improvise your own knife.
Please do not use this unless you have no other choice, as it will likely end in your opponent's death. First, apply the water slick. Secondly, get out of the puddle and away from any conductive surfaces. Thirdly, knock something electric on the floor. Your opponent will be shocked and most likely killed. Unplug the appliance and drain the water or soak it up using towels.