Knowing what the deck is stained with presently could be important. Keep all sawdust out of the pond, especially if the wood is pressure treated. An oil based stain is probably better than water based because it off gasses more quickly. All prep and stain work should be done when there is no threat of precipitation and the pond should be covered in plastic to keep debris and stain from getting into the pond. Pressure washing is better than sanding. If they cover the pond with plastic be sure to put slats across and aerate well. I would check with the manufacturer before applying the stain.

Vicki Vaughan Flat Rock Koi www.flatrockkoi.com