Congrats: The Marlin is a great boat.
It is also a boat you should not attempt to drive right off the bat. I will explain.
For beginners, a Marlin is a lot of boat not only to pilot but to understand. There are way too many variables that will make your first few trips stressful without an experienced person on board with you. Wind, currents, navigational aids and how to read them, docking, the list goes on. Then there are the onboard systems, AC shorepower, head, galley components, etc.
A safe boating course will not show you how to drive that boat. It is a big help, for sure, but a day, or several, on your Grandpa's Marlin with an experienced Captain is the only way to start out. A real Captain on his game will teach you how to dock with twin engines, read channel markers and educate you on all the components aboard. He (or SHE..don't want to get into trouble here...) will help you learn how to trim your boat, use tabs effectively, and teach you how to run the boat in rough conditions.
Once you get the basics, then you can go from there. Your local Grady-White dealer or marina should be able to put you in touch with a Captain for hire as Russ mentioned above. It's worth the money, believe me.
Enjoy!