Theoretical question: Murder in neutral waters by person without citizenship? Will person be prosecuted? What if victim is also a person without citizenship?
Most seafaring nations, including the United States, by express constitutional grant, reserve the right to regulate conduct on the "high seas" (an archaic concept synonymous with what we would now call "international waters"), and could intervene according to naval rules of engagement and admiralty.
This jurisdiction is not limited to crimes against citizens of a state seeking to intervene.
Also, in practice, it is virtually impossible not to have citizenship, or at least, nationality. The fact patterns that can give rise to that are very extreme and arcane. It is quite common not to be able to discern a person's citizenship, but it is very rare for someone to actually not be a citizen of any country.