A fishing spear gun.
Just in case it's land-sharks doing the home invasion.
But seriously, okay just semi-seriously, you might have shampoo in your eyes so you better go with a shotgun. With all the moisture and water you probably want a pump shotgun and you don't want to waste your granddad's Remington 870 so I'm thinking one of those "marine grade" / silver cerakoted Mossberg 500s but make sure to upgrade to plastic/polymer for the foregrip and stock. Hang it high up in the shower, in one of those rust prohibiting plastic bags, with the internals covered with some quality anti moisture rust prohibiting, protecting gun oil
And don't bother going with a pistol grip, because accuracy suffers at a greater rate than most shooters would estimate.
Besides,
think of what will be going through the minds of any bad guys when you burst out of the shower Rockin and Rolling your shotgun!