Selection of presence sensor
There is no one sensor that is perfect, but some are more reliable than others.

Yes, the temperature and humidity can play an important role in your sensing, and has to be taken into account. That's not to say that things like Ultrasound don't have a place in the scheme of things.

There's one word you need to think about here: Correlation.

If one single sensor isn't completely reliable then add a second that works in a different way. If both of them agree that there is someone in the shower (i.e., the sensors correlate) then the probability that there is actually someone in the shower is considerably higher.

The great thing about a shower is you are working in a defined enclosed space. That makes it much easier and there are plenty of sensors which, when combined with others, would be quite good at reliably sensing the presence of someone in that enclosed space.

Some ideas:

Ultrasound above the person's head


Pros: 


Simple to arrange. 
Safe.

Cons: 


Can be fooled by the water from the shower head. 
Steam isn't good for them.



Infra-red distance


Pros:


Accurate
Can be sealed

Cons:


Expensive
Harder to work with
Can be blocked by steam



Horizontal infra-red beam


Pros:


Can be positioned outside the shower glass with a reflector inside
Safe

Cons:


Small sensing area
Can be blocked by steam



Pressure sensor


Pros:


People weigh more than water so is very reliable
Simple to interface

Cons:


Hard to install safely
Needs to be completely waterproof and sealed.
Could be a shock hazard if not waterproof



You notice I haven't mentioned PIR. I'm not a fan of PIR for sensing the presence of humans. They work by detecting moving heat sources. If the person is moving (entering an area, walking round a room, etc) then they get detected. If the person stands still (as is often the case in a shower) it "forgets" they are there.

Of the choices above (and I am sure someone would come up with others) I would probably opt for the infra-red beam. Maybe split it into a V shape to cover more area - that is, the beam enters the side of the shower at an angle, strikes a reflector on the shower wall, and is reflected back at another angle. The detector is then placed away from the emitter. You could also have a second set to give better coverage.

Infra-red distance measuring in/on the ceiling could be used to improve correlation and thus the probability that there is actually someone there. 

I'd stay away from pressure sensing unless you can get hold of a proper waterproof pressure sensing mat designed for in-shower use. Don't try and make one yourself, waterproofing isn't an easy task, especially when people stand on it all the time.