Mouth of Fafnir

The Mouth of Fafnir is a location mentioned multiple times in the Comic, Dangers of the Deep, and is noted to be part of Berk's Territorial Waters.

Geography
The Mouth of Fafnir is an underwater feature in the waters surround Berk. It appears to be a chain of underwater mountains and rock formations, which has an opening, or "mouth". Fish migrate into Berk's water through this channel.

According to Astrid's commentary in the Comic, there are also reefs in the area and close by.

Dangers of the Deep
Alvin the Treacherous seeks to distract most of the able-bodied villagers of Berk by closing off the Mouth of Fafnir with a net. This prevents fish from entering Berk's waters and causes a shortage of fish. While most of the adults venture into the Veil of Mists to find out where the fish went, Alvin the Treacherous attacks a vunerable Berk.

Trivia

 * Fafnir is a dwarf in Norse mythology, who is turned into a dragon because of his greed.