why not both?

That's so sad...

But dolphins breathe air?

But they'd need to stay in the water.

So uhm...
Can it?

Yes. The main cause of death from cocaine use is heart attack or stroke. Cocaine stimulates neurotransmitters which result in the constriction of blood vessels, raising blood pressure and heart rate. Since dolphins are generally more massive than humans, and they are designed to sustain higher blood pressures (and greater variances in blood pressure) in order to dive, it would take much more cocaine to kill a typical dolphin than a typical person.