Without trying something similar yet, here are some ideas:

• motion sensor on top of your shower
• trying to detect the vibration of the water flow
• a simple switch that you hit before you get into the shower

Also think of other times you "use" the shower, like cleaning it. Some methods will also be triggered then.

Multiple waterproof 4k cameras pointed at the inside of your shower from every angle and image recognition by some AI that detects when a naked person enters.

Don't forget to use Wifi and no firewall

Made a flowchart for you:

< Do you feel wet? >  
     |  
     +-- yes --> < are you indoors? >  
     |                |  
     |                +-- yes --> < are you wearing pants? >  
     |                |                      |  
     |                |                      +-- yes --> [ you peed yourself ]  
     |                |                      |  
     |                |                      +-- no --> [ you're taking a shower ]  
     |                |  
     |                +-- no --> [ it's raining ]  
     |  
     +-- no --> [ you are not taking a shower ]  

I bet you can rig up something microphone-based, there's tons of software nowadays for analyzing audio, and you don't need to run the analyzing software on the device with the microphone anyway. Though the analyzing would likely take a little bit longer that you'd want it to.

If you have a shower cubicle, could you put a door open sensor on it, and assume you're showering if the door is closed? I think it's normal to leave the doir open when not in use. It would probably also work if you had a shower curtain, so long as you always pulled it all the way closed in use.

You could use a classic dead man switch: Place several buttons around the whole place - basically everywhere except in the shower. Once you stop pressing any of these buttons for - let's say - 30 seconds you can assume you're taking a shower.

@batshit I tend to use the fact that I'm naked and wet as my cues.

This is a fun question.

I have to assume the absolute ideal would be some sort of water flow detection right in line with the shower head. Likely hard wired. But like I said, that’s an ideal.

I think the other suggestions in the thread are reasonable. The microphone one seems most appealing to me since that could theoretically just occupy an outlet anywhere in the room.

I experimented with a humidity sensor in the shower and was surprised how quickly it spiked. It's how I woud do it. Other option would be a leak sensor in the basin but getting it to turn off at the right time would be very hard. You have a lot more control over this with a humidity sensor.

I'm using zwave for everything like this and wouldn't be at all concerned about battery for either scenario. Both would last years, much like my door and window sensors do.

I would just go with special button, the most straightforward simple solution you have direct control over. Also sometimes you might need to use the shower without mood lights and then system based on humidity or water flow will just be annoyance.

If you use some tools when showering, like brush or spung, you can make a contact sensor into the hook/holder (more work, probably need a custom 3D print), but should also work and you don't need to press anything

Would a battery powered smart switch be an option?

Hit the switch and let HA do a command series.

@batshit I use a derivative of a humidity sensor, it works well enough and is really simple to set up.

I would use something like this to measure water usage in combination with an presence sensor in the shower stall. Water flowing + someone present in shower stall = someone taking a shower.

OK, assuming your switch is also smart

Option 1: long-press/double function turns on both light and fan, that triggers an "I am going to shower" 5 minute delay where a humidity sensor senses when it is over and keeps the fan on until humidity is stabilized with the rest of the house.

Option 2: get a smart flow meter instead of a smart valve. Then you have useful statistics for shower water usage in addition to an instantaneous status. This has the side effect of the mood lighting turning on "late"

Option 3: get one or make an esphome + LD2410S on battery or plugged in close by, nobody goes in the shower for more than like 10 seconds without showering

Option 4: Time of Flight sensor outside of the shower pointing in to tell when the door/curtain is closed. If the ToF sensor is (for example) between 1.1m and 1.2m, the door/curtain is closed

How about a simple on off toggle inside your shower handle? Stupid simple and works anytime the waters turned on.

@batshit Don't you notice getting wet? ;)

Seriously though, couldn't you use a water leak sensor for this? Just make sure if it has a speaker to disconnect that, unless you like a loud beeping found when you shower.

Your individual shower habits might be predictable enough to if than your way to it with existing sensors.

If you track current draw on your boiler / water heater that would indicate a draw on hot water during time when you typically shower. You can add other checks for it to pass if you find too many false positives.

If you’re as glued to your phone as every one else alive atm a simple rfid / wireless charger or something you set your phone on when you strip for the shower triggering the event will likely be the most straight forward approach unless you bring your phone in the shower lol I’m occasionally guilty if the podcast is good enough 😅

If you have a shower door: use a door/window sensor (on the outside). Trigger the mood light if the door was open for only a couple of seconds.

Still more complicated than hitting a switch, tho, IMHO.

@batshit I have 2 humidity sensors: one inside the bathroom and one outside, to calculate the reference hum. point (to adjust triggering on rain or dry weather). I can adjust the speed of detecting by moving the hum sensor (cheap xiaomi ble) near the shower. It triggers in several seconds and turns on the exhaust fan.

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.

@batshit so ein Quatsch