A white horse can be understood as an agent, a person riding a white horse. Generally speaking, this agent is a handsome young man. A dream boat, as its name implies, is something in a dream. It can be said that it is a dream boat, and it can also be said that it is a dream boat drifting in the rivers and lakes.
I have read this novel, too. Based on the novel, my understanding is this: who comes by horse in the dream and who goes by boat?
Don't blame the poor explanation ...