south of the mediah castle

You must talk to Captain Moguly of Splashing Point BEFORE going to this location and do his quest!! While doing the quest you can go to the Sunken Mediah Trader location and get the knowledge. I didn't have to talk to the baby otters, just swim out and find the spot in the water. That completes the quest.

You need to have the mediah questline completed. I'm not too sure how much you need, but I have used a character that has completed only the balenos questline and it didn't give me the quest.