I have an 18 acre lake with tons of this stuff.. it extended s 8-20 feet.

What's the best way to get rid of it? Or most of it?

I have used the granular 2,4-D. Navigate is a good brand, last time I bought it it was about $250 for 50#. Btw what % of the surface acreage is "covered"? How deep is the water and what is the turbidity like?

5 springs feed this lake.. max depth is 18feet... the watershield extends anywhere from 8 to 20 feet or until the depth is over 7this feet

It's too late to use 24D if you want control this year. I just treated an acre pond consumed with watershield and got great control using Clipper. It will burn it out for the rest of the year but will come back next year and you can use Navigate or Rodeo. The Clipper will work in a few weeks because it is a contact herbicide where 24D is systemic and will not be able to kill the root with the remaining growing season 2011. You can buy Clipper Herbicides anywhere on the web. Just google it.