Undead creatures do not require air, food, or sleep. They can cross the ocean directly, walking underwater as if traversing mountains and valleys.
The Death Knight only contemplated for a few minutes before abandoning the idea of crossing the ocean that way—it was too time-consuming, and he didn't have much time to spare.
He was a Death Fear Punisher, bound by the Decaying Garden, traveling through the Death Passage to various planes. He could stay on a plane for at most one day before the Decaying Garden would "summon" him back.
Death Fear Punishers are indeed undead creatures, but they're not mindless killers.
The Death Fear Punisher would travel to different worlds seeking contacts and agents, regularly using fear and bribery to reestablish his authority among them, aiding in the collection and acquisition of intelligence concerning the Impermanence Card and related matters.
