overseas traveler; passenger (on an international voyage)
Refers specifically to a person traveling abroad, especially by ship or air. More formal and narrower than 旅行者.
Many overseas travelers gathered at the airport.
Overseas travelers must present their passports.
乗客 is a passenger on any vehicle (train, bus, plane, etc.), while 渡航者 emphasizes the act of crossing the sea or traveling abroad.
Compound of 渡航 (overseas voyage) + 者 (person).