Nice game. I've been playing it for a whole summer break, to determine the best route. Can't find my notes yet, but I remember Buenos Aires - Marseilles as the most lucrative route, at least when you're using the expensive ships.
Other interesting routes (but not safe ones, so better with low cost ships) are
SanFran - Cape Town
SanFran - Kalkutta (or Karachi?)
Monrovia - anywhere
To Basrah with Weapons
Rio - Sydney
Making quick money:
You should buy 2 big cheap ships, choose Rotterdam or Hamburg or Rio as home port, and travel only to Monrovia for a while.
Or: Buy the small high-class ship, keep it in the docks, and sell it when the price raises.
If this sounds boring to you, choose SanFrancisco or London as home port, and try to stay in the southern hemisphere and asia (low costs and no bad offers)