I can't speak for open sea routes, but having sailed on a sail boat in the mediterranean coast of Spain many years, I can tell that the coastal traffic is more or less organized in corridors set by the authorities. The reasons are varied, safe pleasure navigation around certain coastal villages, fishing banks where trawlers are usually arround at slow speed with huge nets drafting, etc. Sometimes there is only a minimum distance to the coast, sometimes you also have a minimum and a maximum when you go through two restricted areas.
Here is an example of the Gibraltar Strait:
http://bp3.blogger.com/_DG1oRRK1tVs/...Carta+Estrecho