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"
Well if the sea finishes at the mouth of the Helford, what do you call the bit which extends up to Gweek - a leg? a finger? As you say, it is tidal to Gweek. My son was painting a boat in Gweek boatyard today, which pretty well dries out at low tide. I admit my marine terminology is a little shaky (think pointy bit and blunt bit on ships, etc) but I cant see where navigation comes into this? Anyway I was just trying to explain to
On The Top Bunk that whatever you call it, it's not a river in the sense that he quite naturally understood.