Three-drops water radical, side-form of
水. The indexing radical, pure semantic fit:
汪 originally describes a broad expanse of standing water (
汪洋 vast ocean), tears welling (
泪汪汪), or — in modern colloquial use — the bark of a dog
汪汪. Same water-family:
海 sea,
湖 lake,
洋 ocean.
