I've never had any success on watershield with glyphosate herbicides (ie RODEO, AQUAMASTER, AQUANEAT, ERASER, or other "Roundup-type" products). Traditionally, only NAVIGATE/AQUAKLEAN (granular 2,4-Ds) were effective for treating this very hardy and tenacious specie. But, when imazapyr received its aquatic registration, the "silver bullet" was discovered through some subsequent field-trials.
A 1/2% to 3/4% solution of POLARIS or HABITAT combined with a 1/4% solution of a good methylated seed-oil surfactant, applied topically, mid-summer, yields excellent (but slow) control of watershield - and even allows spot-treatments of boat-lanes and other specific sites while leaving other areas for "cover" (if desired). Use a hand-held pump-up, backpack or ATV-type 12-v sprayer for applications.
