氵 is the left-side form of 水 (water). Beaches, shoals and rapids are all defined by their relationship to water — the meaning anchor for 滩 sits squarely with this radical, which links it to a huge family including 河, 海, 湖, 流.
难 (nán) supplies the sound, which has shifted tones to tān. The faint semantic flavour fits too — shoals and rapids are difficult water to navigate. Compare 摊 tān (spread out) and 瘫 tān (paralysed) for the same phonetic series.