What is the action of going aboard a vehicle or starting a journey called?

The action of going aboard a vehicle or starting a journey is referred to as "embark." This term specifically implies the process of entering a vessel, such as a ship or airplane, in preparation for a journey. It emphasizes the transition from land to transportation and is often used in contexts involving travel by air, sea, or even land.

The other terms, while related to travel, have different specific meanings. "Debark" refers to disembarking or getting off a vehicle, which is the opposite of embarking. "Travel" is a broader term encompassing the act of moving from one place to another but does not specifically denote the act of boarding a vehicle. "Navigate" pertains to the process of planning and directing the course of a journey rather than the action of boarding a vehicle. Therefore, "embark" is the most accurate term for the action described in the question.