If I sailed on my boat to international waters, could I earn my living growing cannabis and selling my crop to those in need?
One important question: How are your customers going to reach you? Will someone else set up a ferry service between your boat and the mainland? How will your customers manage to clear customs when they reach the mainland?
ETA: Well, that’s three questions, but they’re sort of related 
Do you have enough guns to ward off the pirates? Or, more importantly, the navies of respective countries who might be interested in shutting you down? You could scream about illegality all you want but who would you turn to?
So no, I’m not aware of any controlling legal authority preventing that. But also no controlling legal authority preventing any world’s navy from attacking you and bringing you in for “justice” despite that doing so might violate international “law”.
You may not be in violation of any laws (or then again, you may), but there are some logistical problems.
Having enough surface area to produce a sufficient amount of crop would be a challenge.
If your customers can find you, so can pirates.
I am living in a utopia, but aware of the laws. I would have cruise ships under the flag of a country who has legalized marijuana only stopping by my ganjaboat. The cruise ends when the ganja is finished.
I suppose an army of Rastafarians could easily be mobilized. The countries would have no jurisdiction out there.
Mobilized? When, and from where? Raised I could see, but you either take said army with you from the get-go, or you have no army. Even with the army, you’re still vulnerable to pirates unless you can sink their ships before they sink yours.
Countries that have a legal drug trade typically don’t like outsiders horning in on it.
See what Cecil says: In international waters, are you beyond the reach of the law? - The Straight Dope
Assuming your ganja boat flew the flag of a country that legalized it and had their permission to do so, then yes. But economically viable…I don’t think so.
They’d smoke your stash before it could be sold 