Just sing aloud in the shower, preferably "YMCA", any intruder will run scared sh@tless.
Seriously though, if I were so concerned with someone invading my house why I am in the shower, I'd just turn the alarm on. As long as you have motion control units throughout the house, you should get enough prior warning to get to your gun.
Now, if you're on the toilet, it becomes interesting... it's not like running out of the shower naked and dripping. Now you have to do some serious soul searching after the fact..