I never use water in my 18 1/2 inch WMS. I foiled a 14 inch Clay Saucer and it sits nicely on the top of the Weber Water Pan, which I also wrap just to keep it looking nice. By foiling the clay saucer it makes for an easier clean up. I change the heavey duty foil about every five cooks or so, or whenever the clay plate starts looking pretty yukky. I have ONLY been able to find a 14 inch clay plate at Home Depot. Lowe's does not have the 14 inch one for some reason, or at least the stores I've tried.
I just bought a second one as a spare, cause sure as shootin' I'll drop it one day and break it. I now have a new one in the waiting. I do NOT put anything in it. I just let the grease drip into it. When I clean it I use some de-greaser in the plate if any has drained down past my lousy job of foiling.
Good luck. If you have a 22 1/2 inch WMS, I'm not sure what size saucer it uses, but it's on this site.