I've cleaned keyboard and mainboards using a regular bathtub. Half filled with warm water using a small amount of mild detergent. Immerse for 10 minutes to soak. Then agitate underwater to wash out all dust. Stubborn marks, like dirt stuck to keycaps, use an old rag or cloth to wipe while item still immersed underwater.
If you didnt use too much detergent, no need to even rinse. But you can just use the shower head to rinse off.
Important, is to reasonably quickly dry it. Dont let the wet sit there for days. I SHAKE off excess water. Then let the thing sit at an angle on a warm radiator so the warm air circulates and dries. One day later it is usually dry.
A hair drier on cool is another option.
SHAKE AGAIN to make sure no water reappears from under connectors or behind keys.
I've done this many times. Regular tap water, regular bathroom in various countries across Europe. Note that the water is considered drinking water, and not that nasty stuff you sometimes get in the states out of the faucet!
Mainboards and keyboards are quite robust. Make sure no batteries of course! And remove fans and their motors.
WARNING: dont do this to your HDD! LOL