Quest objectives: Investigate the passage below the grate
The sewer tunnels in this area of the quest and onward are extremely buggy.
Sometimes when fighting a mob (jellyfish, barracuda) they just disappear into the wall and are unable to be attacked but the game says I am still in combat and do not regenerate health.
Warmaster Forgal Kernsson will also randomly disappear into the walls to fight an invisble mob only to reappear a few minutes later.
Lastly, as a mesmer if you get downed in these sewers and use the clone ability there’s a chance to get teleported into the wall where you cannot move or attack anything because attacks are obstructed. The only way out is to bleed out and retry the story.
EDIT: In the last objective of the quest I was killed and respawned at the checkpoint whereupon the sea creatures appeared at the entrance (see attached) to the sewer which was out of land. When killed though, the conversation triggered and the quest was completed normally.