One way to hold this word: the moment the gangway goes down and people step off the ship onto solid ground. The journey by water is over, and everything and everyone that was inside the vessel now comes out of it. Whether you walk off yourself or the crew brings you and your boxes off, the picture is the same — out of the ship, onto the shore.
From the word’s history
The source gives disembark as dis- + embark: embark is going into the vessel, and dis- undoes it, so disembark is coming back out. The word tree behind embark shows en- (into) plus bark, a ship.
字源資料直接寫 disembark 是 dis- 加 embark。embark 是上船，dis- 把前面那個動作反過來，所以是下船。再往前看，embark 裡可以拆出 en-（進入）和 bark（船），也就是「進到船裡去」。這樣一來，disembark 的核心畫面就是字面本身：從船裡出來。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:637239378:34295): "From dis- + embark", with the etymology tree listing en- + bark for embark
登入就可以看每一支怎麼連回核心畫面。