This site is of disputed antiquity. If you have any information that could help clarify this site's authenticity, please post below or leave a post in the forum.
Apparently these rock markings are best reached from the River Gannel by boat. There are not far from a place known as the Fern pit. This place is easy to find as there is a small ferry that operates from this point across to Crantock beach.
ps
The Gannel is tidal and a dangerous place to swim or wade, in order to walk right around the estuary you will need to avoid high tide.