Yup, that's pondweed alright, I have some in a few ponds I manage, but not even close to that thick. Stuff comes up fairly easy, I'd just manually harvest it as I really avoid use chemical control unless a last resort.
I guess you could consider triploids, but pondweed is on the iffy end of thier preferred spectrum. A big IF. Plus with triploids they can eat all the beneficial plants and leave you with a ruined pond.
Can you drop the water level at this place?
