Early in the game, the easiest way to make money is by beating up groups of pirates and enemies as you explore the game. This is quick to do and will net you a few bucks from each defeated thug. It's not going to allow you to retire early, but it will give you a ready source of cash for small early purchases.

If you want to upgrade your ship and your abilities, you'll need to learn how to quickly earn money in Like A Dragon: Pirate Yakuza in Hawaii.