[Norfleet]

Connecting the lake DIRECTLY to the sea would be a little much, since it's equivalent to teleportation.

Why not make one or more intervening river provinces? Make them small, set them to have no population or sites, and let them swim through those: If the river provinces are rather long and border many land provinces, this also gives Vanheim, CoTS Marignon, and anyone who forges a Pocket Ship the interesting ability to sail the rivers.