Never seen that before. It does look like a but is just scraping a bit of the leaf and moving on, like it didn't care for the taste. Only other thing I can think of is hail damage.

I wouldn't be thinking nutrient, salt issues. Doesn't look like that at all.

Hi Waterbug guy. I wouldn't clean the pond out but I would take quite a few and remove them to another container like a whiskey barrel large pail, put a dissolved fertilizer tab in the water. Use your pond water if you can or make sure you use dechlorinator if its fresh water. It really looks like a sprinkler hit them or some sort of water spray when the leaves were in full sun. Another test is take one and place it in a plastic zipper bag with some pond water. Within 24 hrs, you should visibly see what skirting across the surface and selectively peeling your leaves. Could very well be Caterpillars and yes, they can tread water. If you find that it is Caterpillars, buy some BT worm killer by Ortho. Great stuff, will not harm your fish or anything else. Follow directions on the label and spray in the cool of the evening when the sun is not on your leaves. Caterpillars like to eat at night and you can stop them in their tracts. Good Luck and happy ponding.

Hi
I kept WH for over 20 years and began to have a similar
problem. Got so bad I gave up on them
Are you in florida?? Have heard there is a major eradication program going on using a type of bacteria.
Don't know if that's true BUT I was unable to save mine.
was unable to find any info and since I quit growing them I quit looking Maybe you could find some info from fish/game or dept. agriculture?? gary