氵 is 水 in its left-stacking form and places 汀 firmly in the water family — sandbars, shoals and tidal flats are all where water meets land. Same radical you see in 河, 海, 湖.
丁 gives the sound, with a tone shift from dīng to tīng — a common d-/t- swap in the phonetic series. Same phonetic appears in 钉 (dīng, nail), 顶 (dǐng, top), 订 (dìng, agree).
