Aquatic plants will grow in soil, gravel, even in the crevices between rocks. All they need is something to anchor themselves. They derive their nutrients from the water column. Gravel will not help and over time it will be filled with organics. Hand pulling may or may not work. Some plants propagate readily from pieces of root. I do not know if Water Shield is one. Any pieces of root left may sprout.
Someone else will probably post a better response and solution.
