You would not be able to say "He is in a ship." because there are two people and "he" is not plural. "He" means "a male". You would have to say "They are in a ship". It should also actually "They are on a ship" and not "They are in a ship". Also, "One person is on a ship" would be correct because there is one person on the ship. "Only one person is on a ship" would be wrong.
