Some water will run down in front of the flap. There are no flaps out there that will be 100% water tight but it will keep the debris, rocks etc from damaging the paint.
The front flaps do not need brackets on the 2009-2016 F-350. The flaps will fasten to the lip of the fender with self tapping screws. If you have oversized tires and less the 1/2" of clearance between the tire and wheel well there are custom fit offset brackets for the front tires, https://www.sharptruck.com/truck-hardware-custom-fit-mounting-brackets-gb756685