That's where golf ball divers come in. They retrieve the used golf balls to sell them back to golfers, creating a second market for people who are OK with reusing their golf balls. It takes a certain tenacity to do it, because just like any self-employment, no one is going to pay you if you don't catch any balls, but if you can pull it off, it's basically free money. On top of that, you're saving the environment by recycling. Who knew you could be a hero and make money?
It can be dangerous since golf course water hazards can have creatures in them, and anytime you're going underwater there is room for danger. If you don't believe how lucrative this can be, check out the reality show that's based on people who do this exact same thing.