I figured he'd get the ship later on when he knew what he was up against.

Hey, Angel hired someone to make the suit before the voices were done explaining to him what he had to do. Compared to that, I have no trouble buying that Spike would stumble on a Yellow Submarine and go "Right. That's mine. Might be useful sometime."

ETA: And now I have this image in my head of Spike hearing Angel explaining all this and going "...Right. Y'know, Dru used to talk to birds, too. And stars. And pixies. And the kitchen table. And..."