Just lock random people up without due process! That’ll teach them!
Also, unless I'm very rusty with the legal system, if you're the captain, by definition you're responsible for anyone on board. Coming back to port with anyone missing should be a HUGE problem for you. I don't get that impression here.
After proving ( in the court of law) that it was because captain's neglect. If sailor A kills sailor B just for the fuck of it (and this happens or the true reason might be unknown) do you propose preventively punish a captain before establishing causation and guilt?
And most of injustice happens because our laws are constructed in fucked up ways and almost never punish politicians that are the ultimate reasons for such system. The example - asymmetric laws when it is easy to declare person dead by mistake with all the consequences and then it takes years for a person to get their rights back. And many other laws are like this. You want justice - go after politicians first. Here is the idea - if any person is harmed as the result of such asymmetric laws every politician immediately gets 10% salary cut until the law is fixed. Suddenly you'll find politicians busy doing nothing but fixing law system. This of course will be a problem on its own.
I have better idea. Imprison POTUS or whomever else goes for a similar role in other countries. After all they're the ones ultimately responsible. And do it for any injustice that has lead to a death/s or anything stronger then mere inconvenience.
And that keeps happening...
I'd propose in any subsequent event of a missing observer captain does time, ship seized, company closed and that's done even before police starting asking questions. I bet there wouldn't be any missing observer after that.