W.A. Manatees are rarely sighted unfortunately. They have been seen in the bolongs of Makasutu apparently .....but not by me. Having said that when we stayed on one of the floating lodges at Makasutu in 2005 there was something large and air-breathing in the water outside our lodge one night which was snorting and splashing (there are no hippo at Makasutu).
By the middle of November the Bishops and Whydahs are moulting out of their breeding plumage and most of the intra-African migrants have moved on so Levaillant's, Klaas's and Dideric Cuckoos become scarce as does Woodland Kingfisher. By December these birds have mostly gone and many of the sunbirds are in eclipse plumage. Egyptian Plover is easiest in October to early November but then starts to move back up The Gambia river to their breeding grounds in Senegal - a few linger on around Basse until January.
If I were planning a trip I would go in early November and spend two weeks in total. It really needs about 5-6 days for an upriver trip to Janjangbureh (Georgetown) via Tendaba. Such a trip should yield Egyptian Plover, White-backed Night Heron, African Finfoot, Northern Carmine and Red-throated Bee-eaters, African Pygmy, Giant, Woodland and Blue-breasted Kingfishers as well as good chances of Black Crowned Crane and possibly Abyssinian Ground Hornbill (though the long grass hampers views). A night drive along tracks around Tendaba often yields Standard-winged Nightjar whilst various other good birds are also likely such as Bronze-winged Courser.
A week is needed to explore sites within an hour's drive of the coast such as Marakissa, Pirang/Faraba, Kotu Creek/sewage ponds/ golf course, Abuko, Brufut, Bijilo, etc.
You will need transport and a guide who knows the birding areas well.
A two week trip should yield about 215+ species -many being West African specials (eg Blue-bellied Roller, Guinea & Violet Turacos, Oriole Warbler, Mouse-brown Sunbird, Western Bluebill, Bearded Barbet, Western Grey Plantain-eater, Gambian Puffback, etc ). There are also many species which though widespread African birds are easier to see in The Gambia than in East Africa such as Grasshopper Buzzard, Beaudouin's and Western Banded Snake Eagles, Wahlberg's Eagle, Palm-nut Vulture, Hooded Vulture, Harrier-Hawk, Senegal Thick-knee, African Green Pigeon, Four-banded Sandgrouse, Greyish Eagle Owl, Abyssinian and Broad-billed Rollers, various sunbirds, etc.