A few seconds with a weed burner and a wire brush will clean it right up.
I HAVE built a hot lump fire in the bottom of the WSM and set the waterpan over it to let the gunk burn off, but the weed burner works better. I do like my weed burner...
Short of a weed-burner, Dawn Degreaser spray (in a small royal-blue spray bottle near the dishwashing soap) sprayed on and allowed to soak for a good long while will also cut a lot of it. You can also spray the pan down with cheap ammonia and seal it into a trash bag overnight. That will cut it quite well - just be careful with the fumes.
Best cure is prevention - spray the pan, and particularly the flat lip of the pan, well with non-stick cooking spray before each cooking session. It's amazing how the cooked-on crud will literally just slide right off.
Oh, and I've discovered that if I left an uncleaned and undumped waterpan in the cooker, and it comes a big freeze, and the water and grease freeze in the pan, the whole kit and kaboodle including the burnt on stuff will all slide out in one big hunk!
Other than that, $4 at Academy Sport and Outdoors store will get ya a new waterpan. I must confess that I've replaced my big ones several times when they got too gross and gunky to deal with.
Keri C, still smokin' on Tulsa Time