transparent, clear
湜 centers on visual clarity: water that is transparent and clear, allowing you to see through it without obstruction.
湜 combines 氵 (water) with 是, which likely contributes the sound and may reinforce the idea of correctness or straightness, suggesting water that is clear and true.
Water 氵 that is 是 (correct, straight) is so clear you can see straight through it: transparent water.
For ショク, imagine a shockingly clear pond: shock -> ショク, and the water is so transparent it shocks you.