Stupid monster...

[palistov]

From my experience, water lurkers don't patrol like grunts do. They just sit still and wait for their prey. To avoid your creepy little fishy friend getting stuck, you need to add patrol nodes at its level around your map. These will help the AI navigate said fishy friend around things like rocks, walls, pipes, corners, etc.

Place them with the area tool (press 8) and make sure they're 1-2.5 meters apart. Any further seems to break the link between the nodes, and the monster won't travel between unliked nodes.

In your debug menu, activate the entity info option. While an enemy is active, you'll see the patrol nodes and their linked edge nodes, allowing you to judge whether the monsters path will make sense.

Good look schooling those water lurkers.