I know there is a flap that the nozzel has to push open when you fill your tank, the question I have is do I need to worry about water getting into the cap-less fill tank when I am trying to wash out the area around the filling port.

 

I would not use the high pressure spray with the gas door open. When I wash the car, I keep the gas door closed and then use a detail spray with a cloth around the inside, after the car is dried off. My '11 and '13 Fords had the lockable plug. Why Ford redesigned the filler neck to not accept the plug, on the 2015's, is beyond me.