A few people have asked me for patterns for these types of structures. I think it is much easier to just explain the gist of how to freeform it, rather than give an explicit pattern, so here is my attempt.
Simply begin with a chain, which, at some point, you'll begin single crocheting back into. This will form the point of the leftmost branch. When the sub branch is as long as you would like (three stitches for the first small branch in the diagram above) chain again to start a second sub branch. Single crochet back onto these new chains to complete the sub branch. You can then continue to single crochet along the "stem" of the branch to finish it off, and chain again once you are back to where you'd like the main "stem" to be. (A "stem" is made from chains that remain without single crochets while you start working another chain.) Repeat as desired, and then begin single crocheting back along the stem instead of chaining again when you've finished the branch farthest from your start point. To make new branches on the other side of the main stem, you simply chain, and single crochet as before - but remember to single crochet when you reach the stem.
The "sea grass" here is was worked up in size 10 thread, but it's also fun to make in worsted weight or heavier yarn.You can also try varying the height of stitches (double crochet, slip) that you use to work back into the chains.