Yeah, when people talk about "buffing coastal cities", that's the key point to me. One big reason why I like not giving a separate trade route from a Harbor is that too often, I would simply build a harbor for the trade route. Got a one tile lake? Harbor for a free trade route. It seems... cheap to do that. Now, the flipside, you need to make sure that coastal cities have a reason to exist, and that there's actually a reason to settle them. Even a lot of the above ideas of giving coastal cities a bonus for trade routes, does that apply to any city built on the coast? Any city with a harbor? Does the trade route itself have to go through the coast? Does it have to pass by the harbor?
I would allow unlimited number of trade routes but limit their usefulness.
- Trade routes should cost upkeep.
- Transporting food and production should cost food and production and transport costs. (Take it from one city and give it another city.)
- All trades with AI should be realized with trade routes (e.g. Cash and Carry). This would make piracy/naval blockade more interesting.
- The transport efficiency should depend on type and distance. Trade routes over water should have higher capacity for goods, be faster (until invention of railroad), be cheaper and have bigger range. A short trade route should have a higher capacity than a long one, since the ship/caravan can do the travel more often in the same time.
- Sending multiple trade routes to the same profitable foreign city should result in reduced revenues when traders compete with each other. This should depend on competition for traded goods.
- Markets should have a smaller capacity for number of land trade routes than harbors for sea trade routes. Upgraded markets/harbors should allow an increased number of trade routes.
Historically people wanted coastal cities to participate in global trade and earn money or have access to needed resources, food and luxuries.
also I am against the idea of adding random production to sea tiles. I mean, are people supposed to be building stuff out of fish bones, or what? no, getting only food and some gold out of the normal sea tile is fine, the sea tiles should not be giving the same yields as land tiles. Coast cities and inland cities gotta have some difference.
Until late 19th century whale hunting was common. In modern times players could use underwater/deep-sea mines, harvest abyssal nodules (mangan), etc.