Here are some examples -- there are others.

http://www.marinasdirectory.org/
http://marinas.com/
http://www.marinalife.com/

If you're interested, you can compare and contrast what and how these various sites do "it". Some sites have regional focus (or certainly "regional roots").

They share the commonality that all are intended to sell advertising and listing placement fees. To do so they provide a service.

I suppose authors, song writers, poets, doctors, lawyers, and indian chiefs generally could have the same thing said about them -- except it doesn't always apply, like those web sites.