I think it is also important to take into context the difference between GnK and BNW. In GnK, coastal starts were really really bad. Most people settled in-land and never touched the Maritime and Naval Technologies because there was no need to.
With the power of Cargo Ships, coastal starts are more then encouraged. Since people like to play the historical realism card, this is more realistic as empires and civilizations often prospered due to their cities being along the coast, enabling trade. Plus it promotes naval combat which is often ignored in GnK because there was not much incentive to settle on the coast.(unless you are Carthage)
That sounds good in theory (and I'm sure that was the thinking behind it) but it fails in execution in the game IMO. Since you can have ALL of your trade routes from one city if you want, you really only need to have ONE good coastal city in order to enjoy the benefits.
IMO, this is just totally incorrect. If it were like that in reality, predominantly land-locked countries with a limited sea access like Germany and Austria would have been as much of sea-trading powers as England, Italy, Spain etc (which have FAR more coastline and ports).
I thought they should reduces the trade per route from a single city to encourage people to spread the routes out amongst multiple cities. If that were the case, you'd want to have multiple port cities in order to maximize the trade. But sadly, that's just not the case.
As far as defending the trade goes, well, I still think BNW AI is far too peaceful and it's too easy to sit back with 3-4 Tradition mega-cities and coast to victory. I generally need at best just a few ships for anti-piracy and I'm fine for the entire game. I think I can count on one hand the number of overseas routes I've lost in BNW in about a dozen games (King and Emperor so not at the insanely high levels). So sea route vulnerability to me is more or less a non-issue.
I thought they had a golden opportunity with foreign trade to really make the system shine but IMO, it falls short of the mark. It's a good idea that isn't fully developed IMO and the fact that the 'Fall balance patch' has come and gone with no real changes doesn't fill me with hope that they recognize any deficiencies with the current system let alone have any plans to improve it.