Sea trade have long reach. I think land based trade can reach 15 tiles while sea based trade can reach 30 tiles. Now I do not know how switching work. I would assume that a sea trade route need to stop in a city with a trading post before it can start travel inland and the same with a land based route turing into a sea based route.
Trade post are created in a city after a trade route to that city have been completed.
Each time a trade route stop by in a trade post its supplies are restocked so it can travel an additional 15/30 tiles and if the trade post is in a foreign city the value of the trade route increase by +1 gold.
So if you are doing oversea trade you may first have to set up a trade post in a costal located city (one with harbour) and then you can start to send trade routes inland from that city you have established a trade post in.
You can create very long trade routes if you have a network of trade posts and these trade routes tend to be worth alot of gold because each foreign city with a trade post the trade route enters increase the value by 1 gold.