Making clouds is easy in theory. There are a few ways of doing it, most of which involve spraying sea water in the air -- say with fans, or ultrasonic waves, or a whole bunch of dudes with jet skis. And it could be pretty effective. One study has suggested that only 1,000 of these weird ships would be enough to halt the effects of global warming.
Could It Backfire Horribly?
Oh my goodness, yes.
This is actually one of the tamer methods on this list, largely because it's just messing about with seawater, and we've already got a pretty good handle on what clouds do (angels sit on them). But anything which has a large enough effect to halt global warming could have a large enough effect to do lots of things. We've certainly never done this before, and it's close to impossible to model all the possible consequences. A global reduction in temperature might happen, at the cost of unpredictable and negative local effects. Would all that extra moisture in the air change weather patterns? Mess with the aquatic food chain? Will it confuse the stupid birds?
That problem is going to show up a lot in this article. Basically, the Earth's ecosystem is complicated, and monkeying around with one aspect can have unpredictable effects elsewhere. Admittedly, this particular system should be reversible. If something goes strange, like all the whales up and leave the ocean or cats start laying eggs or something, we could just turn off the spitting boats and hopefully everything would go back to normal in a few days. But it's still a big ask, and seeing as this would effect the entire world, we should probably come to some kind of consensus before doing it.
Getting seven billion people to agree on something shouldn't be hard.