In level 8 "Fear" You can find the hall of voices right by the ending of the level, in the final chamber before the exit head to the right side and navigate through the bottom of the glaciers. You will find a mound of blue coral, approach this and you will get an audio prompt from Ryan asking Sonya if she's hearing this, and at this point you will get the achievement.
Slight disclaimer, this is the most likely area for it as it is where I got the achievement, however I am not 100% sure on the exact trigger location, by following the map provided this is the most likely area, and it's the area I got the achievement as well.
Good luck, and have fun. (The location is marked by the green circle)

Note. I was not the one who made the map, I only added the location for the hall of voices. I got the original map from a very nice guide by ToastBubbles who spent the time to make a comprehensive map of the entire cave system, including montipora locations https://www.trueachievements.com/a288269/submersion-achievement
I would highly recommend checking their guide out.