I really find the Black Hole theory the most plausible and possible, because that would also explain how Sanjuan Wolf travels the Grand Line. He is too big to fit on any ship, and he certainly cant always walk in the ocean next to Blackbead’s ship, given the depth, danger and cold of the deep sea. So I theorize, that whenever the Blackbeard Pirates need to sail, the captain stores Sanjuan Wolf away in his Black Hole