Brian, one factor that you haven't disclosed is "how much" 2-3' water is present within your lake that serves as hospitable habitat for coontail. Actually, both water-depth and water-clarity will determine where coontail seeds are able to survive and thrive post-germination.
The more suitable habitat there is for coontail expansion (beyond an acceptable level), the greater the need to take preemptive measures to curb or control its population before it gets "bad".
IMO, there are better products than diquat for spot-treating (managing) coontail; namely endothall or copper-complex herbicides - and their granular formulations in particular. Diquat has a very short life-span in water; which is good for some very specific situations, but it also limits diquat's efficacy & efficiency when dealing with a high-biomass submerged-weed scenario, especially when subsurface-injections of diquat aren't feasible.
