You're overthinking this
The laws on a ship of any kind where the captain/commanding officer is anticipated to act with autonomy during the voyage always operates under the maritime law of the country whose flag is flown by the ship. It doesn't matter if the ship is military (wherein a naval subset of maritime law is applied) or civilian (wherein a civilian subset of maritime law is applied).
International maritime law has no affect on what the individuals do on their ship. International maritime law is a legal structure that allows nations to resolve disputes when national interests are involved. It would have some claim on individuals stranded on an island claimed by a nation not the flag flown by the ship. In other words, if a Netherlands cruise ship is stranded on one of the U.S. Virgin Islands, the maritime law of the Netherlands operates on the ship, U.S. law operates on the island. International maritime law resolves disputes in procedure and punishment.
You said I was overthinking this, please get to the point...
There are three kinds of law involved in your scenario:
1) The law of the nation whose flag is flown by the ship. This would include corporate ships as, on Earth, no ship flies without a national affiliation (OK, one type of ship does... they're called Pirates).
2) The law of the nation claiming the planet the ship is stranded on or the region of space it is stranded in. This certainly applies to anyone foolish enough to step outside the ship.
3) Treaties are important because law does not bind by its simple existence. If the Zurenians don't recognize Terran law and you had the misfortune of breaking down in Zurenian space, the Zurenians will do what they please, which includes enslaving the ship's population or imposing punishment on ship population. This is what we'd call "international maritime law," which isn't a body of law so much as it is a series of treaties that embody rules for resolving disputes over jurisdiction, claim, and punishment when two (or more) opposing bodies of law collide.
A word about developing law
Laws are only useful when laws can be enforced. A colony ship had better have agreed to what laws they intend to abide by long before they launched because it's completely unreasonable to believe a group of people can conjure up a decent set of laws when they're beating off wild Haruuga beasts or dying from the remarkably mild bite of that purple fuzzy caterpillar thing over there. And it gets really complicated when you meet these short skirt-wearing dudes covered in grey mud who patiently pantomine the fact that you're six weeks late with the rent and you suddenly wonder if it would be easier to ask for permission to commit genocide or forgiveness.
If your universe has so many colonists, such accesible tech, and so many empty planets, that people really can buy their own ship and colonize their own planet without governing influence, then they really need to worry about law first. However, most likely, organized governments will control the colonization process to (a) guarantee they have influence over the laws and trade and (b) collect fees (have you ever heard of a government that doesn't collect fees?).
Keep in mind that governments are created to enforce law and governments are quickly addicted to the habit of doing so. It's difficult to believe colonists would be allowed to develop their own legal system before declaring or being allowed to declare independence from the colony-sanctioning-government. Thus, my point about the three types of law, above.
Finally, a word about those pesky pirates
Do you remember when I said laws are only useful when they can be enforced? Pirates exist where law is difficult to enforce. Even among a group of reasoning beings — scientists and philosophers the lot — if there's no way to enforce the laws you brought with you the dude able to wield the biggest stick will inevitably (and I mean that, inevitably) begin changing the laws. We see that happening with established governments world-wide on Earth today, where individuals and/or groups shout down, bully, threaten, etc., to get laws changed. Sometimes it's for the best, sometimes for the worst, and that's always from someone's point of view. U.S. citizens usually look on our revolutionary war as a great thing. I'm not convinced British subjects agree. I've met Canadians who wish the U.S. would get on with paying them to absorb their country. Other Canadians who would bust a vein screaming over the very thought. And French Canadians who wish both of the above would just set Quebec free already.
My point is, laws are complicated to enforce when it's "easy." Pirates exist when it's not. Generally speaking, people tend to follow the laws they were raised with — but never forget there's a little pirate in everyone's soul, and whether you're later labeled "pirate" or "privateer" depends very much on people's points of view and who wins in the end.
Law... you gotta love it....