Granular 2,4-D will work well, the only issue is getting it to the roots of the plant. If the Watershield is thick enough that it covers the water surface it might not work as well as the liquid Imazapyr.
Al, I agree with your approach.
Is Imazapyr your preferred herbicide for watershield? I used diquat as some of the stuff we buy for the farm was also labeled for aquatic use as well. It did a number on them but many came back. They've been around for years so I suspect it would take several treatments to burn them out with it.
