海岸 and 沿岸: shore walking and coastal land or waters
Compare walking along the shore with seawater sampling off a coast, using the action and target to read coastal land or waters.
Walking along 海岸
海岸（かいがん） refers to land bordering the sea. 「朝早く海岸を散歩した。」 describes taking a walk along the coast early in the morning. 海岸を identifies the place traversed during the walk.
The sentence describes a coastal walk rather than walking on the sea. It does not narrow the coast to a particular sandy beach; its precise terrain requires further information.
A sampling location and a coastal town
沿岸（えんがん） is used for areas near the coast. 「調査船は島の沿岸で海水を採取した。」 says that the research vessel collected seawater off the island's coast. 沿岸で identifies the sampling location rather than describing a walk on shore.
「強い風が沿岸の町に吹きつけた。」 says that strong winds battered the coastal town. Here 町 is a town on land, so 沿岸 is not restricted to places on the sea.
The place noun with its action
The 海岸 walk and the 沿岸 sampling connect different places and actions. 沿岸 can concern coastal land or waters according to context, and the ranges referred to by the two words can overlap.
Seawater sampling is stated, but the vessel's exact distance from shore is not. 散歩した, 採取した, and 町 supply the action or target needed to read the place noun.
In the seawater-sampling sentence and 沿岸の町, does 沿岸 always refer only to waters?
Show the answer and why
No. The first example concerns the vessel's coastal sampling location; the second concerns a town on land near the coast. 沿岸 alone does not fix the reference to only land or only waters.