I would not recommend putting the Weber water pan directly onto hot flame (eg, your propane burner). Although I may be wrong, I don't think the paint/enamel on the water pan will hold up. What I'd worry about more is, water over 130 degrees can scald, and a Weber water pan sloshing around with 130+ hot water may not be safe, especially when lowering it into the middle section which is a bit akward, especially if the charcoal is already going and you're getting smoke in your face. Scalding hot water on your wrists or on your t-shirt (belly) makes for a bad day.
When I've used water in my 22, I usually get a one handled pot going with hot water (about 2 or 3 quarts) and then use that to fill my water pan. I get it pretty hot so that if I need to top off with colder water it the temp doesn't drop too much. The one handled pot allows an easier one armed reach down into the smoker, my favorite 3 qt one handled pot is about 13" from the tip of the handle to opposite side of the pot, so one like that should work on the 18" WSM.
Be Safe & Smoke em' good!