I read someone's opinion on the meaning, and his was as good as any. Slaves being led through a tunnel in which they thought was leading them to freedom. Instead the tunnel opened up at a ship that would bring them to America where they would be sold. "Sea of Joy" being a euphemism for the ship, because at the end of the song Winwood sings "set them free. I don't know if the song means that, but I think of it that way now when I hearvit. My favorite Blind Faith tune, really one of my favorites, period.