You are both correct to some extent.
Ports increase trade base income by 2,3,4,5 respectively by level but do not increase trade fleets
Sea trade increases trade base income by 2,3,4 respectively by level and also increases trade fleets by 1,2,3
Note that I said trade base income. That means that if your region has little trade to begin with, increasing that won't do a lot, so its more profitable for regions with high amounts of tradable resources.
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