do I need to have played Watch Dogs 1 to understand Watch Dogs 2's storyline?
You don't really have to. The basic storyline from the first game is glossed over in the introduction. Otherwise, you don't need to know the storyline of the old game.
The storyline of the sequel so far is much better from what I have played, and you don't need to even use a gun after the prologue stuff due to how the hacking system has improved.