On the surface, it seems a straightforward fitness choice: at 36, and with a body fat percentage that would be the envy of tour cyclists, he has little appetite for the almost 10 teaspoons of sugar that go into the average Coca-Cola can.
But it also smacks of a conscious rethink of how he wields his influence, positioning him as a champion of a healthier lifestyle. In the same way as a 43-year-old Tom Brady has carved a fresh niche evangelising about his daily electrolyte intake, the veteran Ronaldo clearly sees a value in selling the virtues of optimum hydration.
Whatever his motives, Ronaldo has shown this week that he remains a trendsetter without peer.
Barely 24 hours after his anti-Coke performance in Hungary, Paul Pogba was emulating him in Munich, removing a bottle of Heineken from view and placing it on the floor.
The trigger was different: Pogba, as a practising Muslim, appeared not to be fully comfortable with the idea of promoting beer – even if the offending Heineken was the alcohol-free variety.
Ronaldo, clearly, is the trailblazer when it comes to athletes' gesture politics.
A fascinating power struggle looms if this pattern continues. On the one hand, Uefa is understandably protective of its sponsors, having added lucrative Euro 2020 tie-ups with TikTok, Qatar Airways and Vivo. It needs those relationships to flourish if its flagship international tournament is to continue to expand. But on the other hand, it is the event's superstar players on whom the event's corporate cachet ultimately depends.
And if Ronaldo and Pogba indicate that they want nothing to do with Coca-Cola or Heineken, the two longest-standing Euros partners, on whose side will Uefa come down?
Ronaldo is being lauded by anti-obesity campaigners for his stance against Coca-Cola and, by extension, all that junk-food products represent. It is a stretch, though, to depict him as any kind of anti-corporate crusader.
The architecture of self-promotion around Ronaldo is quite bewilderingly intricate, with his image used to hawk everything from online poker to the CR7 selfie app, screen protectors to hair-transplant services.
The notion that he is more concerned with remedying society's ills than with burnishing his own mystique does not, if you will forgive the expression, hold water.
For example, in a 2015 interview with CNN intended to spread the word about his latest range of headphones, he walked out when the reporter dared to ask him about the Fifa corruption scandal. "I don't give a f***," he shot back.
"What do you want me to do? Speak about product, he speaks about Fifa … come on."
Few in the pantheon of modern sporting greats have sung for their suppers quite as dutifully as Ronaldo. It is his self-mythologising, coupled with his social-media profile, that explains why he collects an estimated £970,000 for every Instagram message. In the past fortnight, he has used the platform not to offer any telling character insights, but to provide mentions of a live scoring app and a hotel that he is soon opening in Madrid.
As a public-health advocate, he may yet discover his most powerful voice yet.
Before his Coca-Cola-for-water stunt, he was advertising a book by his friend, Dr Pinto Coelho, on the secret to a healthier immune system. For when you support Ronaldo to extremes, you are buying not merely into an athlete but into his entire way of life.
You can wear his fragrance, visit his museum, subscribe to his gym, even sleep in a monogrammed CR7 blanket. So powerfully has his latest gesture resonated, it will not be long before you are drinking his own branded water, too. For where Ronaldo goes, everyone else follows.
