Tom, make sure it really is milfoil and not something else. I would wait until the water temps are in the 60's before treating it. Ideally treat it in the morning on a sunny day and when the forcast calls for sunny days for the next few days. Here is a guide to submerged plant identification, look at the coontail picture as well. Can you post a picture of it? I know that Navigate works extremely well on milfoil with lower doses but does not work near as well on coontail.

http://aquaplant.tamu.edu/database/index/plant_id_submerged_plants.htm