"alligators sunned themselves side by side"
Crocodylus niloticus in German East Africa (c. 1906-18)
Creative Commons Attribution Share AlikeCrocodylus niloticus in German East Africa (c. 1906-18) - Credit: Bundesarchiv, Bild 105-DOA0965 / Walther Dobbertin / CC-BY-SA

Marlow (or perhaps Conrad, as the same error is made in his 1897 short story 'An Outpost of Progress') is confusing the crocodile with the alligator, since the latter (indigenous only to America and China) is not found in Africa.

The Congo River is home to Crocodylus niloticus (the Nile or Common crocodile), as well as Crocodylus suchus (the desert, Northwest African, or West African crocodile), a species distantly related to (and often confused with) the Nile crocodile.