Early in Venice's history they were in a location with bad, swampy land, few natural resources (including even fresh water), and had to get almost everything they needed to live by trade.
To have something to trade they boiled saltwater in giant pots and sold the salt.
Was Venice unique in this or did everyone along a coast do it? If Venice was unique then why?
Also, what else did they sell early on?
Frederic C. Lane covers this in-depth in his books about the subject. It mostly had to do with the Lombard embargo on their vassals selling salt to "Greeks", and so, since Venice was officially a Byzantine vassal, they had an exclusive monopoly to sell salt to Constantinople.
To be honest, though, even from the early days salt wasn't as big as shipbuilding.
I apologise if my answer is off, however I feel that 'early' is kinda subjective. Venice success was mainly due to its industrial development. Venice's shipbuilding industry expanded greatly due to its growth in trade. The Arsenal was expanded when the demand for great galleys increased in the 13th century. Venice's growth in trade also resulted in the Venetians gaining more wealth. This led to an increase in the demand for goods which promoted the growth of manufacturing industries, like glassware, candles and soap. The printing industry also develop due to Venice good trade connections and thus able to serve a large reading public. All this created more jobs for people in Venice and hence attracted people from other parts to settle in, making Venice a place with many Talents around the world.