I remember not having any trouble with this quest, so lets check some things before giving up:

- you already said that you have Truth of the Ocean turned on, so we can skip that
- did you caught all the Ichthyosaurs without leaving the area? You dont need to catch them back-to-back, but in one sitting, so to speak.
- the "Unspoiled teeming waters" will appear a bit to the side, not amongst the normal nodes - did you ever see a message about a "swimming shadow" near by?
- just in case you hit the Teeming Waters already: did you make sure to switch from larger to normal?

...maybe it is bugged and re-accepting it will fix it - could still be user error aswell, though...