Plain English Meaning
A stretch of water set up for seaplanes to take off and land from, with lights at night so pilots can see where the safe landing area is and where the hazards are.
Full Definition
Civilian water airports — areas of water designated for the takeoff and landing of seaplanes — that are marked at night by a standard pattern of yellow and white lights. Yellow lights mark the perimeter of the usable landing area, and white lights identify obstructions and hazards within or adjacent to that area.
Context Anchor
Seen in discussions of airport lighting and beacon colors, especially when identifying types at night.