I've read criticism that it was illegal based on examples like a Nazi U-boat captain killing survivors of a torpedoed vessel, but the Allies routinely straffed Japanese troops after sinking their ships. So what's the deal?

It was wrong then and it is wrong now.

America should be looking at why its people require drugs in the first place, why do they want to ruin their lives, why do parents fail to educate their children about the harm of drugs. Stopping the supply will not stop the despair.

It's not up to us to decide whether or not it was justified. We have no say in what Donald Trump does. We have enough problems with Albanese.

But, if you want an in-passing reply, my answer is yes. Drug runners are absolute scum, whose activities put them beyond the law and decency. Just because such summary justice hasn't occurred before, doesn't mean it is not right.

The US, any country, has the right to protect itself from any outsiders intending to do harm to its citizens. The now corrupt, weak and ineffectual legal system in most Western countries means that tougher measures need to be employed.

Snivelling about rough and ready dealing with criminals is just another Western weakness.

America should be looking at why its people require drugs in the first place,
Yuyutsu,
That applies particularly to Australia the Number one country per capita of drug abuse.
And, yes the shooting is justified !
Also, medical support for overdoses should only be provided if private health insurance pays for it.

It may not have been illegal under international law for a NAZI U boat captain to machine gun survivors of torpedoed vessels during WWII, but it became a part of international law following WWII in 1949.

1949 Second Geneva Convention, Article 12: States that wounded, sick, and shipwrecked persons at sea must be "respected and protected" and treated humanely, regardless of the cause of the shipwreck.
http://ihl-databases.icrc.org/en/ihl-treaties/gcii-1949/article-12/commentary/2017

1977 Additional Protocol I, Article 42: Prohibits attacking a person parachuting from an aircraft in distress.
Exception: This protection does not apply to persons parachuting from aircraft who are clearly engaging in hostile acts upon reaching the ground.

Was the US justified in killing the survivors of the Venezuelan drug boat sinking?
No, it wasn't even justified in attacking them.
This is what the Coast Guard is for.

You need to look at it this way.
The United States talks about Rule of Law, and a Rules Based Order.
Let's begin with the idea of due process, if you are accused of a crime, you get your day in court.
Even if the drug trafficker was caught red handed with the drugs in American waters, he'd still be afforded his day in court.

What Trumps doing,
This would be extrajudicial killing, but it's not even in US waters.
They even conducted a double tap strike on survivors, which is another separate crime.
And if you go through the U.S. own government documentation, they say that Venezuela is not a Narco State
The whole thing about drugs is fabricated to provide a cover story to overthrow Maduro.

That's what we get from the U.S. Department of War Crimes.