The soft drinks giant is introducing the new water product by asking the public to choose which ad should promote it.
Shearer and a spokesman from Saatchi & Saatchi Milan appeared on national TV to introduce the executions their agencies had created for the new drink.
Nitro, which handles the Aquarius brand across Europe, is battling it out with a spot called "stay fluid", which is competing against Saatchi & Saatchi Rome's "Aquarius musical" ad.
If the trial is successful, Coca-Cola has said it might use the voting method in other markets.
A Coca-Cola spokeswoman said: "Right now, it's just happening in Italy but it's very much a look and learn exercise. We will evaluate whether we will roll the idea out if it is a success."
The ads launched this week and will run back-to-back for the next two months. Viewers will be encouraged to visit a website to log their vote for the winning spot.
Since launch night on 3 July, Coca-Cola said the website has had more than 40,000 hits.
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