along the coast; coastal route
Used as a noun or adverbially to describe a path or movement that follows the coastline. Often appears with verbs like 歩く, 走る, 行く.
I walked along the coast to the next town.
This road continues along the coast.
沿岸 is a more formal term for 'coastal area' or 'littoral region', often used in news or official contexts, whereas 海岸伝い is a descriptive, everyday expression for traveling along the shore.
Compound of 海岸 (coast) and 伝い (along, following), from the verb 伝う (to go along).