“
The Great Lake (which is really a Scottish loch, apparently freshwater and landlocked) never did develop as a portal to other seas or rivers, although the appearance of the Durmstrang ship from its depths in Goblet of Fire hints at the fact that if you are travelling by an enchanted craft, you might be able to take a magical shortcut to other waterways. Giant squid genuinely exist, though they are most mysterious creatures. Although their extraordinary bodies have been washed up all over the world, it was not until 2006 that a live giant squid was captured on film by Muggles. I strongly suspect them of having magical powers.
”
”
J.K. Rowling (From the Wizarding Archive (Volume 2): Curated Writing from the World of Harry Potter)