If it's just congealed fat floating on the water, toss as much of it out as you can and wash with Dawn detergent and water.
If it's burnt and crusted on, soak it until it loosens up. I use a cheap 5 in 1 paint scraper tool that has been bent just right so it matches the curve of the WSM stuff, and I scrape the really crusty stuff with it.
The water bowl is porcelain coated too, and is pretty durable.
My Boys: Joseph (EI); Andrew (EE); Daniel (EE)
My Toys: 10 WSMs (B, C, DH, DA, EZ, DZ, EO, DU, DR, EE/DU mutt); Brinkman Cimmaron; Chuckwagon Cooker; 4 22.5" Bar-B-Q-Kettles; 2 18.5" Bar-B-Q-Kettle; 1 Red OTS (H); 3 WGAs (EZ, DE, N); 3 SJPs (DT, DR, DR); 2 Smokey Joes (A, K); Genesis Silver C (DU).
"For when democracy becomes tyranny, those of us with rifles still get to vote"