Use a camera. People would love that. Point is, would you want to see a strange device looking at you when you're nekkid?
By saying that you want to MAP where people are, does that suggest this a big communal shower? You need to be a bit more clear in your description.
PIR or IR would be fine just for detecting a presence in one spot. Automatic sinks, toilets, urinals, use plain infrared, while the bathroom lights use pyroelectric infrared. Mapping is a whole different project that will involve a camera or an obscene grid of sensors just to know where a naked person is standing in a room.