The Trade Fleets are goblin fleets. They are mentioned, alongside their leader Vice Admiral Grezzlik, by Gazlowe saying that trade princes invested a lot of money into the Trade Fleets. They are also mentioned in the goblin intro cinematic.
In the RPG
Fleets of vessels (both merchant and military), the goblin Trade Fleets run goods all over Azeroth, and are the trade princes' true power. Each fleet has its own captain, while Vice Admiral Grezzlik oversees all fleets.
About 200 years before the First War, the beginnings of Trade Fleets sailed out of Kezan and into the world. Goblins, always the opportunists, chose war as the perfect opportunity to cash in some gold and began building their trade empire during the First War.
A captain runs a single ship, and some run entire Trade Fleets under a trade prince. Admirals are assigned to run more than one Trade Fleet. Occasionally a captain runs his own private merchant vessel, though he still pays taxes to the trade princes.
The Trade Fleets are outfitted with the most fanatical and destructive creatures on Azeroth. The average person wonders why the trade princes require such powerful armies, if they are holding to their neutrality stance. It certainly seems excessive. The simple truth is, the forces are not excessive. The life of those living on the South Seas is harsh, due to attacks from everything from pirates to rogue magical beasts.
The Trade Fleets require extreme guards. Undermine's goods sail everywhere, especially into hostile or desolate territories to make a coin. Simply put, goblins go to dangerous places. Without strong guards, there would be no trade, because there would be only dead traders.