I've been taking cod liver oil (not fermented one) for about 2 months now but primarily for my skin problems. However, I noticed that my teeth also whitened and strengthened. I haven't been to the dentist lately but I think it's the effect of the cod liver oil since it has good amount of vitamin a and d (especially the liquid one).
Green Pasture’s Fermented Cod Liver Oil is one of the popular ones but they had a controversy about their cod liver oil being rancid. You can read about it here: https://chriskresser.com/important-update-on-cod-liver-oil/
Garden of Life’s Icelandic Cod Liver Oil seems to be a popular alternative to Green Pasture though.
There was a report written by Kayla Daniel, supposedly exposing a lot of scandal relating to the Green Pasture cod liver oil. Taken at face value, it was quite damning. But on closer examination, most of it didn't hold up.
I think it's true that the fermentation doesn't agree with everybody and that some people do better on a non fermented oil. But for those it agrees with, the fermented does appear to be more effective.
We addressed some of the concerns here: http://www.naturefoods.co.nz/cod-report