1.
water's edge, waterfront, shoreline
The point where water meets land — the edge of a lake, river, or sea.
2.
border, point of entry (for control measures)
Figuratively, the point at which something is stopped before it enters or spreads further. Used especially for border control, infection prevention, and crisis management.
Forms
水 (water) + 際 (edge, boundary)
SENSE 2 (border control):
Became widely used during pandemic discussions. 水際対策 refers to measures taken at national borders (airports, ports) to prevent unwanted things from entering the country — diseases, illegal goods, pests, etc. The metaphor is of stopping something at the water's edge before it reaches land.