The Helford River is in fact the name of the arm of the sea which cuts across the Lizard Peninsula almost as far as Culdrose, so not a river as such. Part of it is known as Frenchman's Creek, made famous in the Daphne du Maurier novel.
I hope your navigation in the air is better than that. The Helford River is one of the most beautiful sailing areas on the Sout Coast, but, though tidal all the way up to Gweek, is not "an arm of the sea"