One of the popular species of seahorses is the Weedy Sea Dragon. It can blend in very well to the surroundings. In fact, you may have seen them but thought you were just viewing various types of weeds floating around in the body of water.
The Southern coastline and the Western regions of Australia are home to the Weedy Sea Dragon. Some portions of Tasmania also feature them. The are often found living among the seaweed beds. They blend in with them very well though so you will have to look closely at them.