I used to dive in southern Finland. Our waters are littered with shipwrecks and our team was investigating them and looking for new ones. Our waters are not salty enough for Teredo Navalis ( wood eating sea worm ) so the wooden ships stay pretty intact.
When the waters got too polluted with algae and visibility dropped to near zero I stopped diving in Finland. It was fun while it lasted and I got to see 20+ shipwrecks. We did not dive for treasures, but for the history and the experience. I did find one gold ring though. It was from a ship that sunk in 1880.
This map will give you an idea of the amount of shipwrecks here:
http://maps.hylyt.net/Hylyt/WreckMap.html 