This seems rather difficult to classify and when it is obscurely conducted it boils down to homicide. Maduro may have the identities of the people who died. The UN could validate Maduro’s facts.
But, this is a bit bizarre. The USA Coast Guard has the right to board boats within USA waters (frequently half subs) and remove any of the illegal substance headed toward the USA. This power paradigm is very different. This can be noted as in use when a ship is boarded to remove military weapons, especially Iranian missiles headed for the Houthis. But, this wasn’t on a trajectory to other terrorists.
The proof of the illegal cargo lies at the bottom of the sea in ashes. That is reckless.
Hegseth has no immunity. Trump touts his lawlessness as permission from the Supreme Court. The USA Supreme Court decisions do not carry weight in such dealings internationally without treaties.