Stone radical on the left fixes the substance: a reef is fundamentally rock — submerged or barely-visible rock that ships dread. The radical anchors 砂 sand, 砖 brick, 碎 shatter and the wider mineral family.
Right side supplies the sound: jiāo → jiāo (exact match). 焦 pictures a bird over fire, and the 'burnt-out, exposed' feeling lingers as a memory hook for the bleached, sun-bleached coral rock that emerges above the waterline.
