From the picture and the fact that you mention this is on your beach, I suspect this pant is scouring rush which spreads through underground rhizomes. Copy and paste into your web browser the url below to see the answer to a similar question which contains several links to web sites. I can not tell from the picture you sent me whether this is horsetail (a look alike) so read the descriptions of the two to verify it is scouring rush. Also included are suggestions for management options. Since this plant is not easy to eradicate and you are finding it by water, you may want to contact a professional pesticide applicator who is knowledgeable on products that can be used by water.
Hope that helps!