In the estuary of the Ogoué, the fresh and dark water of the river do not mix with the salty waters of the gulf of guinea. You can clearly see the frontier between the two, especially from above (you can also see the shade of the helicopter on the water).
It is also the place to be when you try to fish.
And if you plan to dive, once you have crossed the fresh water layer (around 7 meters), the water is clear again but there is no more light. Funny impression when diving a wreck.