For the seaweed I used various embroidery stitches as well as a used drier sheet that I painted with metallic olive green paint. I then fussy cut a branched form out of the sheet and then used a green thread to sitch down the center of each branch.
In this picture you can see a small yellow snail shell that is part of a collection that I gathered from a beach in Maine when I was 4 years old (that was in the early 1960's). There to the right of it is a small white clam and to the left a yellow jingle shell. I was able to drill holes into all of these so I could stitch them down safely.