headland, cape, spit, promontory
岬 centers on a piece of land that juts out into water: a headland, cape, spit, or promontory.
岬 combines 山 (mountain) with 甲, which likely contributes the sound and may suggest a hard shell or ridge extending into the sea.
A mountain 山 beside a shell-like shape 甲: imagine a mountain ridge that extends into the ocean like a hard shell, forming a cape.
For コウ, picture a cape shaped like a coat hanger: coat -> コウ, and the hanger's hook juts out like a headland.