When do you unlock naval trade in assassins creed 3?
Solution 1:
About an hour after asking this, I found out. You need to do Lance the Woodworker's homestead mission where he goes to Boston for his tools. After that, the recipe will be unlocked and you can craft naval convoy. Then you can do naval trade. Hurrah.
Solution 2:
I think you need to craft a ship then you can trade you can craft in your homestead or in any economy trader in the logbook