The grand bazaar research just mentions the second resource as something you can toggle, regardless of whether you toggle it in dock or market and regardless of whether a boat or land trader. But in reality, the boats are unchangeable. You just get food as the “third” resource. This makes sense. But the tool-tips and info is not clear about that.
Thanks @JohnDresty! I think I know what you mean. I’ll see what we can do about being clearer here. Appreciate the report!