I found these resources.
I'd love to hear about your pond and if these work
out for you!

http://www.wetwebmedia.com/BrackishS...bracplants.htm - for
underwater plants

and

Narrow-Leaf Cattail Shallow Water Pond Plant - tolerates brackish
water.
Found here ~
http://www.pondmarket.com/store/inde...tegory_id= 29

I'd also look at your state's web pages - native plants of Georgia,
brackish water
http://georgiawildlife.dnr.state.ga....187&txtPage=11

Widgeongrass Ruppia?maritima
Saltmarsh Bulrush Scirpus robustus
Dwarf Spikerush Eleocharis spp.

k :-)