In truth, the ocean is pretty lawless. "International law" is a nicety and little else. Treaties work between countries, but have no bearing on individuals. The countries sign on, and at most, might pass domestic regulations to comply -- then, in your average third world country, said regulations are promptly ignored. Without an enforcement body with actual coercive powers, i.e. a "world police", you basically need the consent of the defendant to do anything. Violate those treaties and .... well, maybe there'll be trade sanctions or diplomatic fallout on an international level if you have a country that's completely ignoring them. But for any individual actor, there's no Law & Order: Mid-Pacific Ocean to respond.

Maritime law is a real thing, but it requires a party to step up and take responsibility for its administration. "Flag of convenience" countries don't usually have the infrastructure or the inclination to pick up the actual labour of enforcing laws against their registrants (who are usually nowhere near said country -- imagine the burden the Liberians would have if they had to actually go out and enforce stuff against their civilian armada). Other countries may step up and voluntarily assume jurisdiction based on proximity or, in cases of contract, be designated by the parties as the country of jurisdiction (i.e. the UK was frequently chosen for issues of maritime insurance and claims at sea).

But yeah, shooting a fellow in the centre of the Atlantic Ocean and you're going to have a hell of a time finding a country that can or would accept jurisdiction. With a crime scene that falls into "not my problem" territory, you'd have a hard time establishing any country's jurisdiction on a territorial basis for anyone to investigate. I posted once about the crash of a Boeing 727 that fell into the same problem -- any party that should have investigated shrugged it off, since the presumed accident scene was too far out to be any country's problem.