cape, spit, promontory
碕 centers on a piece of land that juts out into water: a cape, spit, or promontory. The meanings all describe a narrow coastal projection.
碕 combines 石 (stone) with 奇, which likely contributes the sound and may relate to uneven or projecting shapes. The exact historical development is uncertain.
The left side is stone 石, and the right side 奇 suggests something unusual or projecting. Picture a strange rocky outcrop stretching into the sea: a cape or promontory.
For キ, imagine a key-shaped cape on a map: key -> キ, and the stone promontory looks just like it.