You will find marinas just about anyplace that has water suitable for sailing.
This is a list Marinas.com, World's Marinas Portal, and Marina Directory
Yes many marinas are near or in cities. I live in Madison, CT on Long Island Sound about two hours from NYC. We have probably 20 marinas with in a hours drive. I really should count them some time.
When you are old enough to get a job their is no reason you can't get one near enough to a marina so you can have a boat.
The challenge is finding a marina that will let you live on the boat. But there are a few. I know of only one in my neighborhood.