The Void is the open, bottomless water beyond the edge of the playable crust shelf. It contains no resources, no points of interest, and no content. Its only purpose is to keep you on the map.
The boundary
Swim past the world’s edge and the seabed simply falls away into black water. There is nothing to find — by design.
Enforcement
The Void is patrolled by Shiver Leviathans, which hunt in packs and can one-shot an unprotected player. They do not break off and they do not let you wander out and back.
The Void is a soft wall with teeth. The correct interaction with it is to turn around.