With keywords like this, how could I fail to find the book? But I did...
I think this was a Penguin/Puffin paperback book in the 1970s, or possibly even in the 1960s.
Fanch is about 10 years old, lives in the south of Brittany, and spends most of his time sailing his little dinghy around on the Morbihan inland sea, which has very strong tidal currents and takes real seamanship. I think he's an orphan. He gets involved with a gold statuette of Gilgamesh which has been stolen from a museum by art thieves.