What if the gas cap is open in the car wash?

Most cars have a small metal flap kind of like a trap door thst you push as you insert the nozzle. If present, I suspect very little if any water got into tank. The unit that sucks up the fuel from the tank does not sit directly on the bottom of the tank. usually it is about an inch off the bottom so debris or small amounts of water that collect lay on the bottom. All gas tanks have small amounts of water in them due to condensation. For some reason I do not feel you are going to have a problem. If concerned at all, put a couple bottles of 'dry gas" in the tank. It is designed to break up water in ver small molecules and actually burn away through engine. Also prevents freezing. the extreme option would be to drain the tank some. Some? tanks have a drain plug on the bottom. Since water lays on the very bottom of tank as fuel floats on water, you would only need to drain a very small amount of fuel.