Navigate (granular 2,4-D ester) is sometimes effective on coontail, but I've found that early-season treatment are critical due to the product's mode-of-action. 2,4-D is far more effective on young/actively-growing "susceptible weeds" than on mature weeds. The same concept applies to fluridone, although it has more wriggle-room on timing than does 2,4-D.
Possibly the most target-area-specific treatment for coontail is a copper herbicide applied in conjunction with diquat; although the effect is more akin to a chemical-haircut (both are non-systemic in action, and therefore do not move within the treated plant's vascular system) - which isn't as critical for controlling coontail since it isn't a rooted plant.
When attempting spot-treatments, be aware that weeds from adjacent (untreated) areas may re-infest the targeted area fairly quickly.
Personally, I'm more of a fan of Aquathol or fluridone for coontail and many other problematic submerged species. These products are generally easier to apply and usually offer better cost-effectiveness than other products that may appear "cheaper".
A point to be made is that one should avoid assessing treatment-costs based simply on the respective cost of the products. Instead, treatment-cost comparisons should be assessed on a per-month-of-control basis. An "expensive" treatment that costs twice as much as a less expensive treatment may prove far more "cost-effective" if it offers weed-control that lasts 4X longer than the "cheaper" treatment.
Then again, there are many situations where unique site-variables will determine which product(s) will or will not perform - regardless of their costs.
