。
My stool is watery.
Literal
Stool [topic-が] water's like is.
便 here means stool/feces — one of several meanings of this character (also 'convenience' in 便利, 'mail' in 便り, 'flight' in 便名). ~のようです is the resemblance pattern: 'is like water.' A practical sentence for describing symptoms at a clinic.