Mucor, Marble, Mother
This was probably obvious to most, but it only occurred to me on re-reading OBW. Prior to the opening, Mucor had made contact with the Mother, and as the story opens, Mucor (and probably Marble) have become Mother-worshippers, out on their little island. Horn says this pretty explicitly, of his first meeting: a fourth presence on the island (not counting Babbie), explained by considering an island as a mountain reaching down into the sea.
This is, I guess, the reason the fish jump for Horn, at acolyte Mucor's intercession, and the reason why sailors come to consult the pair from time to time. I imagine the Mother's sending Seawrack to Horn is also due in part to inormation provided by Mucor. Seawrack appears to Horn kind of trumped up like an expensive whore, which would fit with Mucor's limited experiences from Blood's mansion.
Remora I think knows. He speaks to Horn in New Viron in terms suggesting that there are indeed gods on Blue; that perhaps on the one hand the VP were scourged by these gods; and on the other, perhaps the VP have retreated to the ocean. He points out that Horn's single prayer on his way from Lizard to New Viron was efficacious: his loneliness was relieved, by the appearance of the leatherskin, and a wind was given to him. I think that perhaps Remora expelled Mucor & Marble, when he learned of Mucor's dealings with the Mother. SilkHorn's admonition at the wedding sermon in RTTW - to respect & fear, but not worship the Mother; and to worship the Outsider above all - is I think directed particularly at Remora. Theocratically, it's a way for the community to avoid the "Scylla & Charybdis" dilemma of worship for the (powerful, efficacious) Goddess, versus worship for a reinstated Pas at some time in the future.
A question which bothers me about this: If Mucor communicates with the Mother, why does SilkHorn need to Have the intercession of Great Scylla on Urth to regain Seawrack? Perhaps it's just that a direct request needs to go from "sister" to "sister" (we know these beings are not bound completely by space or time). Or perhaps "the favor" granted by Great Scylla is more than just Seawrack?
Another question: Why doesn't SilkHorn meet with Mucor at the end of RTTW? What happens to her? Obviously, he has had communication with Mucor's astral form, so perhaps a physical meeting is unnecessary, but it still seems odd.