There's only a single plank between you and hell; a tiny margin separates safety from danger, especially on a boat or similarly risky places.
嵐の海で働く漁師を見ていると、板子一枚下は地獄という言葉が浮かぶ。
Watching fishermen in a stormy sea, the saying 'there's only a plank between you and hell' comes to mind.
ビルの外壁作業は危険だ。足場が崩れれば板子一枚下は地獄だ。
Working on a building exterior is dangerous. If the scaffolding collapses, there's only a plank between you and hell.