The marriage between the Doge and the sea takes place on Ascension day (Thursday, the 40th day of easter) and sports a grandiose ceremony. After mass at St. Mark's, the doge would sail in his ship to the inlet that connected the lagoon and the Adriatic, followed by a precession of the nobility and guilds of the city. There, he'd stop the ship and empty a flask of holy water over the bow. Finally, he would throw a ring of gold into the water while reciting the vows, |"We espouse thee, O sea, as a sign of true and perpetual dominion."
According to the book Venice: Pure City by Peter Ackroyd, the marriage between the Sea and the Doge of Venice could be symbolic of several things. One such is that the ring signified fertility, so the festival in question would be celebrated as a fertility festival. Another way to see it is that the marriage was an attempt to placate the waters which the Venetian relied on for their wellbeing. There is also the theory that the ceremony was a form of rune divination, where the ring is thrown into the waters in leu of runes.
Goethe saw the tradition as a sort of communion: the boat of the doge being the "monstrance" (or dish holding the Eucharist) and the ring being the Eucharist itself, in a sense cleansing, purifying, and sanctifying the waters of Venice.
As for your question, I have no credentials to prove my qualifications or expertise in this area. However, I personally agree with you that the custom of marrying the sea is rooted in pre-Chiristian notions.
People have inhabited the marshy lagoon of Venice since at least the 8th century BC. Subsequent settlers in the 4th and 5th century found protection from the Huns and germanic invaders among the reeds and made their living off the fish and salt. In 421, legend has it, the city of Venice was founded.
Seeing as how Constantine the Great popularized and legalized Christianity around 313, the refugees of the Italian countryside, would perhaps have had Christian traditions alongside pagan celebrations. Since the first Doge of Venice was anointed thus in 697, chances are he was a Christian but still subject to the mixed traditions of the region.
Overall, the custom seems to derive from the Venetians' respect and fear of the sea, being both their fortune and their biggest threat. I can see how that translated into a Christian celebration but I don't know of any specific precursory tradition.
Sorry if I didn't completely answer your question or give concrete evidence (I've only the one book for reference on Venice) but I thoroughly enjoyed the question!
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