Three-drops water radical on the left - side-form of 水, three abbreviated waves. A 潭 is a deep pool or pond, originally the name of a specific river in Wuling. Same family: 河 river, 湖 lake, 渊 abyss, 池 pond - standing or flowing water.
phonetic
覃tán
deep; extending
Right 覃 supplies the sound tán with no shift - a tidy phonetic. 覃 itself means 'deep, extensive, reaching down,' which is conceptually perfect for a deep pool. One of those phono-semantic compounds where the phonetic also carries the meaning: 'water that goes deep' is exactly what 潭 names.
