narrow strip of water
Rare literary expression, similar in meaning to 一衣帯水 (いちいたいすい). Used to describe a narrow body of water separating two places, often with a poetic or classical feel.
See also: 一衣帯水
「一葦の水」, like 一衣帯水, is an old-fashioned expression for a narrow waterway.
The exact derivation is uncertain; the expression appears to be a rare variant of 一衣帯水, possibly influenced by classical Chinese poetry.