The Nexus is a magical transportation hub used for Fast Travel. It is first introduced in the quest The Order of the Grook, and it connects several locations throughout the world.
The Pirate Cove teleporter can glitch and teleport you under the dock. Remember to swim to surface before you drown.
Although there appear to be a lot of crops in the Nexus, none of them can be farmed.
Mage Island actually has two ways to get into the Nexus. Although the Boat Captain says to go back to the dock to re-enter the Nexus, going through the Mage Island secret tunnel at 895 -2888 is much faster.