I think (I am not sure about it) for the land trade you need a road connecting trading cities/civ, while for the sea trade it is just enough to have two ports and a water connection (i.e. discovered sea tiles, linking two ports that want to trade).
That way, "going Da Gama" may be more effective than the traditional "Marco Polo approach" in that in the first instance you would just need two ports and a ship, while in the second your workers will have to build a road from Europe to China for ages.
Anyway,
for this idea, Firaxis!