Yes, Loch Ness had a natural opening to the sea. It's called the River Ness, and in 565 AD Saint Columba allegedly had a run-in with Nessie on its shores. Our Scottish lass would have a far more difficult time navigating the river today, what with all the locks.

If you meant the long-rumoured underwater tunnel, then no, it hasn't been found yet.

As for the American counterpart ... don't forget Lake Champlain, which has a resident creature, also. Locals call it "Champ."