So I did a quick first test with an old 6-DOF Crazyflie Nano we had laying around

. I connected to it, put it down in tap water and...
1. As soon as the antenna and PCB got under water the radio connection was lost.
2. About a second later the red led turned on (battery low indication or it froze in that state)
3. Two seconds after that the red led turned off. Only the blue led lit now.
4. Took it out of the water and the red led started blinking again.
5. Power toggled it and it showed self test error.
6. Could already see small bubbles around the battery connector pins on the bottom side of the PCB. Unplugged the battery and cleaned the PCB.
7. Turned it on, it worked and flew again however it will probably start corroding after some time if I don't clean it properly (alcohol or distilled water)
So the next step. I will try to water protect it with lacquer but that I have to acquire first so maybe next week.