I have just recently cruised. Did a lot of research. On Carnival you will not be permitted to vape in your cabin or on your balcony. Its a $250 fine. You will be treated the same as a smoker and will have to use the smoking areas for your vaping. If you decide to go stealth be sure there is no odor or large clouds coming from your device. I found the areas very convenient, so it was not a problem. Enjoy your cruise.
Those are the same rules I follow for hotels. No to low clouds, try to bathroom vape if I want a little more cloud, and don't set off the smoke detector. There really is no need to blow clouds in places it would be obvious. Discrete works. Save the clouds for the permitted areas and have a wonderful vacation.
As to other substances, I can understand them following federal law. It's not like most of the states like CO face an ocean, and even then, it's federal jurisdiction offshore.
Think of it this way. If most of the passengers will be upset at a behavior that is commited by a small number, they're going to make the majority happy. But as with all things, you can't make everyone happy. And as to recreational stuff, who really believes that no one will notice the odor, and who wants to take the risk of disembarking in another nation where (granted it's usually pretty lax for cruise ship passengers but still) you go through some form of customs to enter and you're really going to bring your party favors with you to hang out on the beach?
Besides, it's probably cheaper to just buy it on the beach for the day. It's not like it's all that challenging on most of the islands typically visited.