Getting from Naples to Venice (or vice-versa) can be a relatively short trip, or can involve the better part of an entire day (or night), depending on the method of travel. Assuming that one wants to travel directly between these two cities, without stopping anywhere en-route, there are essentially two options: the plane or the train. Let's take a look at both.
THE PLANE
There are five daily non-stop flights: two on each of Alitalia and Easyjet, and one on Meridiana. The current lowest fare on Alitalia is 48.39 Euros. Fares on the two low-cost airlines depend on whether you have checked luggage, and its weight. Assuming ONE checked bag weighing 21 kilos, the fare on Meridiana is 48.37 Euros; on Easy Jet, 59.99 Euros. But simply lighten your bag by one kilo, and you'll save 21 Euros on Meridiana and 27 Euros on Easy Jet. And eliminate the checked bag, and you save an additional 7 Euros on Meridiana and 11 Euros on Easy Jet. But add more checked bags, and the fare quickly increases, particularly with more weight. So, it's difficult to easily compare fares because of the luggage factor, and you really need to BE CAREFUL about the weight.
While flight time is under 90 minutes, you have to factor in an additional 60 minutes commute time to and from both airports (longer if taking the Alilaguna boat service in Venice), 60-90 minutes for check-in and security clearance, and perhaps another 30 minutes to retrieve checked luggage. That makes the total TRAVEL time by air closer to FOUR HOURS, and even longer when poor weather delays flights.