deep water, deep
潭 centers on deep water: a deep pool or abyss, and by extension the quality of being deep.
潭 combines 氵 (water) with 覃, which likely contributes the sound and may carry a sense of extending or spreading, suggesting deep, extended water.
Water 氵 beside 覃, which looks like a deep container with a lid (西 over 曰 and 十). Picture a deep pool of water held in a large, covered basin.
For タン, imagine a deep tan-colored pool of water: tan -> タン, and the water is so deep it looks dark and tan.