Specifically refers to a person who boards or is on a ship. Not used for trains, planes, or other vehicles.
All passengers on the ferry were safe.
Please write your name on the passenger list.
Also means 'ship passenger', but 乗船者 is more common in official contexts like passenger lists.
Compound of 乗船 (じょうせん, 'boarding a ship') + 者 (しゃ, 'person').