I've been fighting lake weed and millfoil for over 20 years. Back "in the day" I used a pelletized 2-4-D that worked well although a bit pricy. Several years ago those pellets apparently changed. I spent $300 (elsewhere) and it looked like I had fertilized my unwanted vegetation.
I tried the Opti Amine and it works. I recently bought my second bottle so that I have enough, and have it here for when I need it again. I had to make 2 applications and it does take 6-8 weeks for full effect.
This is the most cost effective product I've found (that actually works).