I have the 2009 18.5" WSM, and it has the big, deep H2O pan. I cooked over 16 hours (butts followed by beans) on it last weekend w/o refilling the H20. Still had a bit left too.
I filled my charcoal ring like in the pic above on the few cooks I've done, and no problem fitting it together. Maybe you were just too generous on the charcoal and went above the ring.