The Venetian trading fleet are ships from Giacomo Garibaldi that will visit your warehouses and trade goods with you. The fleet starts with one ship with a budget of about 5250 gold per port call.
The trading fleet can be upgraded by buying the Venetian Merchant Fleet attainment, which when upgraded twice to the second rank provides up to 3 ships with a budget of 24,000 gold per port call.
Keeping the attainment at Rank 2 is preferable to Rank 3. Rank 3 has way longer spawn time of 45 minutes, compared to Rank 2 spawn time of 20 minutes. Through Rank 2 trade slightly less per port call, over time, Rank 2 will still trade more.
The trading fleet can't be attacked in any way. You can try normal fire, letter of marque, powder keg or boarding crew, but it doesn't work.
Using letter of marque near Rank 3 fleet(Large Trademan's Cog) attract them to attack the ship using letter of marque. After which, the Large Trademan's Cog will remain in situ, and not continue on their trade route.
In the beginning these ships are very useful because you don't have enough ships yourself to export/import products. See the Trade page for more information.