Surprisingly, yes! It might seem banal, but it's actually important to distinguish two kinds of walls constructed in Venice: sea-walls to protect densely built areas from the sea, and defensive fortifications.

Sea-walls have proven to be a fairly recent necessity. For much of the city's history, Venice and its lagoon was much less built-up than it is today: construction was limited to the most solid parts of the lagoon's islands, and low density allowed for even the highest tides to quickly filter away causing little damage. If you're interested, you can read about Venice's early history and development in this answer of mine from this morning. Over the centuries the city became denser, and new constructions began being built on sandy or muddy ground. This, together with the emergence narrow stone streets lined with workshops and offices, exponentially increased the damage that could be caused by standing water. Between 1744 and 1782, the thin barrier islands dividing the Venetian Lagoon with the Adriatic Sea were outfitted with stone barriers, to protect the lagoon from excessively high tides that could roll over the thin sandbars.

Defensive fortifications have a longer history. As early as the late 8th century, the Doge Maurizio Galbaio constructed a church dedicated to St. Peter and a small castle on the eastern end of the Rialtine Islands in the center of the lagoon (the islands which today most people would recognize as "Venice"). Before then, the "Rio Alto" were sandbanks of sediment from the estuary of the River Piave. Unlike the thriving communities of Malamocco and Eraclea, they were home to only a chapel dedicated to Saints Bacchus and Sergius, a small chapel dedicated to Saint James farther west (San Giacomo) and presumably a few farms that constituted a hamlet called Olivolo. The construction of Galbaio's castle would give the neighborhood a name that endures to this day: Castello. As the town grew, the harbors and canals east of the Castello would become the heart of the Venetian military-industrial complex complete with docks and depots, and in later centuries barracks and manufactories, while the west became the heart of the lagoon's commercial life. By the 10th century, the communities between the parishes of of St. Pater and St. James had become more or less contiguous, and were referred to as a single Civitas Rivo Alti. Some sort of fortification seems to have existed enclosing the area between the Castle to the modern-day Arsenal, which itself started off as what we would recognize as another small castle. While the modern Arsenal, epicenter of the Republic's military might in later centuries, retains some vestigial battlements, any and all fortifications guarding the city itself must have been dismantled through the centuries as the city became denser and shipping traffic moved away from the cramped Grand Canal to the modern basin of St. Mark. In subsequent centuries the Doge's castle too would be modified beyond recognition and become what today is called the Ducal Palace.

Of course, evidence of these fortifications, both in sources and archeology, is somewhat unsatisfying because the Venetians never really took to building lasting fortresses in the lagoon. The reason certainly wasn't because they didn't know how; the Venetians constructed imposing fortresses protecting their trade outposts abroad, and since their earliest days maintained a system of watch-towers to monitor river traffic in and out of the the lagoon. However, it was never lost on them that the lagoon itself was their greatest defensive asset. This too, was evident since the Republic's inception. Pepin, Charlemagne's son and successor as King of Italy, attempted to force the people of the lagoons to submit to his rule in the early ninth century, forcing the lagoons to take up arms and defend themselves for the first time since the departure of the Byzantines. How effective the armed citizenry could be against the landed-warrior-levy of the Franks is up for speculation: a decisive battle was never fought. Pulling up buys and markers so that only the most experienced navigators could sail in the shallow waters of the lagoon, the Venetians limited Pepin to the islands of Chioggia and Pellestrina in the southern lagoon the communities of Grado, Jesolo and Eraclea to the north. Once the King arrived in the channel of Malamocco, he found sharp stakes driven into it, making easy crossing impossible. The Venetians loaded onto boats and pelted the Franks with arrows as they tried to navigate the waters (and when the Fraknks were looking particularly short on supplies the Venetians would even throw bread at them). Eventually, Pepin negotiated to have a large tribute paid to him and left. Pepin's year-long siege had the effect of accelerating the urbanization of the Rialto, as people living in communities along the edges of the lagoon took refuge on the Realtine Islands.

So clearly, in the tumultuous post-Carolingian pseudo-crisis, it certainly helped that the communities of Malamocco, Eraclea and Grado were each separated by the mainland by lagoons up to 500km^(2) of brackish water. Sure, the lagoons are only waist-deep in most places, but approaching an enemy over nearly two hundred square miles of open terrain while loudly splashing is possibly the worst way to start a battle. Plus, the lagoons are crisscrossed by channels of varying depth, so traveling in a straight line is never feasible.

The script would repeat itself in the tenth century when a Hungarian horde, itself contracted by the King of Italy to attack his rivals, rampaged across northern Italy and eventually set its sights on Venice. The Doge Pietro Tribuno spared no time into preparing the populace for war: as Pepin had done the previous century, the Hungarians rapidly made their way up the island of Pellestrina, only to be halted at the channel of Malamocco. There they attempted to cross on coracles, which were easily rammed by the Venetian ships storming down the straight at full sail. The victory was quick and compete.

So in conclusion, I suppose it's important to point out that Venice's republican government and organization predates the constitution of the city as we would recognize it, allowing the people of the lagoon spent a lot of time avoiding conflict, relying on their ingenuity and their city's geography to save their skins. This meant that construction of fortifications was never really a priority.