It won't help you now, but I had a good year using Matador and have now had two good years using Intimidator right behind the planter. Leave a skip with the sprayer and the water hemp explodes in it, so I know it's working. Was using the Authority products prior with less than stellar results. Warrant has worked as an in crop pre for me, but it doesn't seem very popular on here.  Without rain to activate it, all of the above is like pissing in the wind.

With water hemp basically germinating all the way through the growing season now, there really is no good affordable silver bullet.

Good luck, and share your recipe if you get it knocked out.