This is the fresh water spring in the Casamance River that the Manatees come to at low tide. Search the site for Senegal to find Cheryl Antonucci’s trip report with the Oceanic Society.
There is also a tour company that offers 110cc scooters to tour from Dakar to southern Senegal and Guinea-Bissau. Their normal route includes a night on Carabane Island which is about 10 miles from Pointe-Saint-Georges and they say they can arrange a visit to the springs. They already have a boat to get to Carabane Island so it seems pretty simple.
Their group trips are not necessarily hard core wildlife trips but it looks like a lot of fun (if you are comfortable riding a scooter). A lot of what I have read about West Africa has been positive.
The 8 day trip on December 9, 2018 is definitely going to run and has open places; I am thinking about it if I can find a decent air fare to Dakar.