Your perception is correct. You're only "killing" the leaf, since WEEDTRINE D (diquat) is not a systemic herbicide (meaning that it doesn't move within the plant's vascular system).
As previously posted, I (too) have found RENOVATE to be comparatively effective on watershield. It is a systemic herbicide, providing fairly decent root control of susceptable plants.
HABITAT (a new aquatic herbicide) also shows a lot of promise on watershield - even moreso than RENOVATE.
With either product, a surfactant (wetting agent) must be tank-mixed with the herbicide.
