You don't. You need to take great precautions crossing any stream, river, ditch, or creek when carrying a loaded gun. If you have a partner, let him cross first, and then hand the guns to him one at a time.
If you are alone, then un-prime the gun( take the priming powder out of a flintlock) and put a toothpick or other plug in the vent hole. Then put some kind of condom, or wrap over the muzzle of the gun. I have a tampion made of wood that fits the muzzle of my fowler, for instance. That, and a rubber band, or properly sized O-ring, to provide a tight seal will keep water out of the barrel if the gun is dunked.
If I had to cross a stream alone, I would be using a walking stick- even if I have to cut a tree down on site to make it--- to help me steady myself when crossing. The gun would be slung over my shoulder using a "hasty sling" I carry in my daypack. Keep your knees unlocked and legs bent, so that your weight remains on the rear foot while you find a flat, sturdy, flat place to place the front foot in the streambed. Only after shifting the walking stick forward, do you shift your weight from the rear foot to the front foot. But keep your knees unlocked all the time, so that your weight remains NOW on the forward foot while you bring the rear foot forward, to find a flat, sturdy place in the streambed to place that foot.
