I would go for a simple humidity sensor. You just need to do a bit of data collection first to get a baseline of "what taking a shower usually looks like" and then based on that create the automation.

The only problem with that is that depending on how accurate you want it to be, you may have to wait a while to take a few measurements until it triggers the automation.

For something like mood lights I would just use a ZigBee switch if I'm being honest though.

If it's an electric shower, you can do what a salamander pump does and add some detection wires on the shower box's on-switch. I appreciate you don't want wires around the shower but it's a fully direct "is the shower on" detector and the wires are technically in the shower rather than around it.

Motion sensor or light switch.

There are so many variables at play here.

sounds like you could use a clapper

Any reason a flow sensor wouldn't work?

I'd settle for the small lag with a humidity sensor. If you're the type to run the shower for a minute before getting in, it should kick in in that time.

Could do a presence sensor or motion sensor pointed at the shower. Haven't played with either, and you'd have to figure out water resistance.

@batshit so ein Quatsch