One possibility is that these stone posts were used for the netting of fish, with the net suspended between the posts and tied off on the landward post for easy retrieval. This was, until recently, quite a common way of fishing and is still used in some areas.
This photo http://www.themodernantiquarian.com/post/49306 shows that the posts are still being used for fishing, the buoys have a pot beneath them as can be seen here http://www.themodernantiquarian.com/post/49308.
It's a great set up, no boat required and fresh lobster or crab for tea.