From the label:
"Do not apply directly to water. Take care to minimize the incidental overspray along the shoreline when applying to terrestrial plants at the water’s edge or to water in areas where surface water is present."
"This product should be applied only when the potential for drift to adjacent sensitive areas (e.g., residential areas, bodies of water, non-target crops, and other plants) is minimal (e.g., when wind is blowing away from the sensitive areas)."
"Control of Terrestrial Weeds Near and Up to the Water’s Edge
"Milestone can be used to treat terrestrial weeds that extend up to the
water’s edge. Do not apply directly to water. This product must not be
used to treat vegetation standing in the water. When controlling terrestrial
weed species near and up to the water’s edge, take precautions to
minimize incidental overspray to the adjacent water. Consult local public
water control authorities before applying this product near public waters.
Permits may be required to treat such areas. Apply the specified rate
(listed in Table 2) of Milestone as a coarse low-pressure spray as ground
broadcast or spot applications. Do not apply aerially for control of weeds
growing at or near the water's edge. Spray volume should be sufficient
to uniformly cover foliage. Increase the spray volume to ensure thorough
and uniform coverage when target vegetation is tall and/or dense. It is
also permissible to treat target weeds within dry non-irrigation ditches and
seasonally dry transitional areas between upland and lowland sites (such
as flood plains, deltas, marshes, prairie potholes, or vernal pools) but only
at times when those sites are dry and are forecasted or managed by water
control systems to remain dry for at least 2 weeks following application."https://s3-us-west-1.amazonaws.com/agrian-cg-fs1-production/pdfs/Milestone_Label1p.pdf