I have once in an airport toilet, but I couldn't finish.
I missed a flight due to my stupidity so I was stressed out and the next flight was 6 hours away so I was bored. I thought rubbing one out would help with both. I went into a stall, sat down and started jerking off. The seated position is not the most comfortable position for me to cum. The stall was not that private with a lot of wide gaps and it was also a fairly busy bathroom with people lining up for stalls. I just couldn't finish.
Gym showers, however, are a different story ;-)