IMO, working for a marina is the least useful step you could take.
With regard to getting a license, you've got a lot of on-board experience to get first before you can get a license. You need to be on the water, on a boat, for 360 days, 90 days over the past 3 years. And that's not living on the boat while in a marina or on an anchor, but actually sailing/driving/crewing on the boat while it is moving.
Requirements for Six-pack lic.
There are a lot more important things to do before considering whether you want to get a captain's license.
I would say that 95% or more of sailors do not have a license, whether the most liberal, a six-pack, or something higher.
Learn how to sail.
Learn the Rules of the Road.
Learn how to navigate.
Learn the basics of maintaining a boat.
Learn how to safely sail offshore.
After learning to sail on a small boat, learning to sail on a big boat would be next, and going that route, crewing is the easiest, and most important step, IMO. Join a local yacht club, volunteer as crew for their races, gain experience and respect from the boats' owners. You could start crewing much sooner than 5 years.
Check out our Crewfinder forums.
Good luck and fair winds,