Here’s how to get quick infinite money in GTA 5:
There’s a boat in this position and it came with scuba gear:
And here’s the position of the treasure chest (purple waypoint) for the fast unlimited money:
Go to the waypoint by boat, and once you’ve reached the waypoint, all you need to do is to jump out of the boat and then search for a submarine below. Inside the submarine, there will be a glowing briefcase, pick it up and you will get $25,000.
Now how to pick it up unlimited time? What you need to do is quickly change character and then change back to the underwater character near the briefcase, and magically the briefcase will reappear. You don’t really need the scuba gear though, but it helps to make your character stays underwater.