Mik, do you have a tractor or ATV?

My approach is to clamp an 8 or 9' pole to my tractor. I wrap old bath towels around the end of the pole, leaving them hanging down about a foot. I saturate the towels with strong glyphosate solution (or use any herbicide you wish). Drive around the pond, dragging the towels over the weed tops.

Using this method, your grass is spared, and you get practically zero chemical in your water. 3 applications a year will keep you weed free year around. It takes less herbicide than spraying, too.