open sea
灘 centers on a wide, open expanse of sea, often with a sense of roughness or difficulty implied by its component 難.
灘 combines 氵 (water) with 難, which likely contributes both sound and the idea of difficulty, suggesting rough or treacherous open waters.
Water 氵 plus difficulty 難: picture a vast, rough open sea that is hard to navigate.
For タン, imagine a tan-colored open sea under the sun: tan -> タン, and the water stretches endlessly.
open sea