I'm guessing something like 1982 UNCLOS might have banned it worldwide, but even prior to that, it doesn't seem like a USN CO in 1970 for example would have been able to execute crew members (yes I know the US didn't sign UNCLOS). And interesting data point might be the last documented example of this happening?
In the case of Spain, the death penalty was abolished in 1978, via the Spanish Constitution, that says this in article 15:
[...] The death penalty is abolished, except for what the military penal laws establish for times of war.
That exception was suppressed in 1995, with the passing of the Organic Law 11/1995, which finally abolished the death penalty for crimes committed by the military in times of war. Those crimes, as specified by the Military Code of Justice, were military rebellion, treason, espionage, sabotage, and war crimes.
In 1978, the Royal Decree-Law 45/78 modified the Military Code of Justice, the Penal and Procedural Law for Aerial Navigation, and the Penal and Disciplinary Merchant Law, substituted the death penalty for 30-year sentences, but exceptuating the times of war. This modification was necessary in order to accommodate the aforementioned laws to the Spanish Constitution.
So, long story short: in 1978 the death penalty was abolished except for times of war. In 1995, that exception was shot down.