AFAIK, all sovereign states law claims to their citizens (and residents and slaves and whatever) and exert their own laws over them--unless they are in another sovereign state's territory. And when a commercial vessel is flagged (titled, or in the US, "documented") in a sovereign territory, the rules of that territory bind it when it is in international waters, or airspace. That's by tradition as well as the laws of the various nations.
That's all general but unless there are specific conditions or variations from a sovereign state or treaty obligations...that's it. Holland America Line is now a subsidiary of the Carnival Cruise corporation, which is based in Seattle. But most or all of the fleet is still flagged in the Netherlands (some were based in other places but I think those are all gone now) so the ship would generally follow Dutch law when in international waters. Contract responsibilities, i.e. to provide due care and a safe environment to the passengers, might follow US law since Carnival is a US corporation.
But the dive operator will be governed by laws in wherever the dive was. And the question of their responsibility versus Carnival's, or Holland America's... Your local attorney will NOT have a solid grasp on the venue issues and those will be critical.
Likewise the waiver's importance or validity will depend on exactly what it said (the attorney will require a copy) as well as the local laws governing such waivers, which could also vary depending on whether any degree of negligence (gross, ordinary, or intentionally ignoring a known issue, etc.) is involved. Again, that will vary with the location.
And then there's the question of exactly what happened, which will be "he said she said" in some places, while in others it will be presumed that the instructor "fiddling" with gear was responsible. Under US law we're taught "guilty until proven innocent" but if the incident took place somewhere that has a French legacy and follows the Napoleonic Code...it often is "Guilty unless proven otherwise".
There are too many crucial factors involved. As others have said, get a damned good attorney who has some grasp of what will be required. Don't be surprised if they refer you to someone more experienced than they are. Don't be surprised if they file suit against all parties, and let the parties slug it out. Do expect this will not be a simple "you only pay us if we win" case.
And again, as others have said, do not have any contact with any parties except your attorney. Since attorneys know how to go online and do searches as well...I would suggest not posting any comments that might be taken as an admission or statement of anything material in the matter. (Which means just about everything.) I expect the attorney will tell you the same thing.
DAN is between a rock and a hard place, they have to work with the various dive agencies. So while they're a great source of information (medical and otherwise) again, don't expect them to say that what any agency teaches or requires is or isn't proper or safe.
