Gee, what's with you guys!
Imagine this OK.
The fuselage is like very round OK (as in "circular") in cross section (that's like a big slice of sausage, get the picture?).
Ok, now when it rains, where does the water go that lands on top of the airplane? It like goes AROUND the "circular" fuselage, as in sticking to the side, flowing down, running down, in like little riverlets of little water streams.
Ok, now imagine in the rain that the skybridge is at the gate, it's slowly being moved toward the door, slowly pressing in, slowing surrounding the door. Now imagine if the little rain gutter wasn't there. It would be like walking through a little waterfall to enter the plane's door, from the little riverlets of rain water flowing down and around the side of the plane. You can see this right?
Now for the 757, "potable" means "fit to drink". Imagine the little riverlets of rain water, flowing down and around the fuselage, running into the hose connections while loading the drinkable water. It would be like, you know, getting the water off of your car after you washed it, into your canteen.
Gee you guys....