It's fine to wash it down with fresh water as long as it is totally dry when you pack it up. Put it this way, It's going to do less harm than saltwater would. This is what one kite manual says,
"• A coating that results from using the kite
in salt water will not damage the kite.
However, rinsing in fresh water and drying
can’t hurt.
• Let the kite dry away from sun and wind.
• Failure to dry the kite after use, or rinsing
in fresh water can cause mildew,
which is unsightly but will not affect the
performance of the canopy.
• Be sure to prevent water, dirt and sand
from entering the bladders when using or
rinsing the kite.
• Failure to dry the kite after use or rinsing
can cause bleeding of dye from one panel
of cloth to another."