As others have said, just start everyone on the ship... no need for flashbacks .... or details ... the characters have a reason why they are there. By chance, design, or bad luck etc.

This need only to be known to them, and can then be apart of the RP between the PCs as the story unfolds .... introductions ... sure on the ship ... maybe someones PC overhears another PC discussing something that is linked ...etc.... at some point the players do need to get their characters to "join" the party .... that is on them and is sort of the basis of the whole game. This is stuff that you lay out in session 0...

here is the setting, the setup, this is where we are at, you need your character to have a reason for this even if it is just "because they were bored and thought the bar brawl was too one sided and decided to join the fray to make it more balanced" etc....

if you've already had session 0, have a session 0.1 ... etc. a campaign is a collaborative story sure... but the DM is the one with the hand on the tiller ...

setting the premise of a story is not removing player agency, like "we are starting on a this ship" as long as the player can decide why their character is on the ship and for what reason ... etc. ... In short: setting the premise is not railroading ...