I am sorry to hear that the achievement has not unlocked. I can confirm that there are no known issues with the achievement unlocking as other players have successfully unlocked it.
I did however have the same issue myself when trying to unlock this one and I had to go through each location again to make sure I had definitely done them all and in my case I was missing two that I hadn't done.
You should be able to find a guide online that shows all of the underwater locations.