I'd suggest Aquathol K for American pondweed (Potamogeton nodosus); applied topically to the surface of infested areas. Unlike fluridone, this treatment normally allows the selective control of targeted "patches" of the plant, rather than fluridone's all-or-none approach.
Reward (diquat, which is non-systemic) will give American pondweed, and other rooted species, a "chemical haircut" - but they'll often return with a vengence.
