subtidal zone
Technical term in marine ecology for the area below the low-tide mark that is always submerged.
A variety of seaweeds inhabit the subtidal zone.
Compound of 潮 (tide) + 下 (below) + 帯 (zone). The reading しおかたい is a straightforward combination of the kun'yomi of 潮 and the on'yomi of 下帯.