What is the origin of smashing a bottle of champagne on a ship to christen it?
I don’t know.
But as a WAG, offerings have long been made to gods or spirits. My guess is that the champagne is an offering to the sea-gods or the spirit of the ship to ensure safe journeys. Why champagne? I’ll guess that the “birth” of a ship is a festive occasion, and that champagne is a festive drink.
I bet it’s because no one really likes champagne. If you are going to waste precious liquor on ceremony, champagne is the obvious choice. 
So, it’s a promotional move on the part of the champagne industry? 
Yes, just like New Years. And little do most people know that sexual human reproduction was actually started up by cigar companies. 
Blood sacrifices are frowned upon of late.
Red wine had been proven to be acceptable to most of the gods in lieu of blood.
Champagne is considered to be a more ceremonial wine among Westerners today than merlot.
(The naming ceremonies of both the Clownfish and SeaBiscuit involved home-brewed beer, and neither of them have attracted the wrath of Poseidon, therefore I conclude the gods are more interested in the sincerity of the ritual rather than the actual physical components.)
All I could google was this