White Water Snowflakes 'Nymphoides indica' is a free flowering frilly white flower that will spread across your pond.  It blooms from it leaf axils rather than from the running stem as a yellow snowflake does.  It grows to 12-24 inches or more wide in water that is 4-24 inches deep and is hardy from Zones 8-11. 

This is an ideal plant also for those having container water gardens.