This isn't quite your average get rich quick scheme.
There are a ton of opportunities to get money in Watch Dogs 2, whether through doing missions and side quests; but in addition to that, the best way to get money in the game is good old fashioned robbery.
If you've spent more than 10 minutes playing Watch Dogs 2 after the pretty epic opening scene, then you know you can hack people's bank accounts and wire yourself money. Yes this move is somewhat scumbaggy, emptying people of everything you have, but hey you're a revolutionary, and you've got things to do!
There are certain people on the streets that have larger than average bank accounts, and the game actually gives you the ability to find out who they are quickly—if you get the right skills.
In the second tier of skills in the Social Engineering tree, you can find a skill called Improved Profiler. The description is accurate: you will automatically see people with stacks of cash in the world so that you can do a little high tech purse snatching. You're like the Robin Hood of hackers—only instead of the poor, the money becomes property of Dedsec.
But hey you're not completely greedy. There's a moment in an early side mission where you wire $20 million to Leukemia Research on behalf of the Watch Dogs 2 equivalent of Martin Shkreli aka Pharmabro while pretending to be Kanye West. This scenario is as hilarious as it sounds.
To spot the whales that you can relieve of their deep coffers, simply walk (or drive) around and look for the blue square around them. While you can't do it while driving, if walking you'll be able to hack them for their life savings. It might not feel good, but it gets the job done.