Well, François, you know, it would depend on the wood... There are large wood stakes in at least one place I know in Brittany that have been planted into the sea floor to be used as mooring poles for small fishing boats, and those stakes are covered by salt ocean water half the time, almost to the top, and they have been there since... the 6th century AD!
(Yes, you read correctly, the 6th century, not the 16th...)
This is another one of the Kerloas Menhir, the tallest standing stone still standing. It is located not far from Brest in central Brittany. A much taller one was in the Morbihan (southern Brittany) but was struck down and broken in two by lightning.