This bug is in the Flooded Tunnels area of the Haunted Zeklos Keep
It happened when myself and one other player, through a very bad series of decisions, ended up in the flooded tunnels without waterbreathing.
what happened exactly is that when we went down with water breathing, we were able to talk to the Aquatic Remnants and get our air back. Since we had waterbreathing, I assumed that this was a feature in case you wound up down there without knowing about the water. When we later made the immensely poor decision to go back in without waterbreathing, we were unable to speak with the Remnants.
To recreate the bug, I believe you just have to try to talk to an npc underwater. Though I'm not certain.
A very simple thing we realized is that in the NWN engine, it is likely that you can't interact with NPCs while underwater without the water breathing spell. This defeats the purpose of having the NPCs there.