Start by using the Grapnel Gun #1 to avoid falling into the water and reaching the roof of the Gotham City Police Dept. Head west, towards the crash site. After getting there, it will turn out the crash site is observed by four snipers #2 who you will of course have to eliminate.
For starters, I'd suggest taking care of the snipers south-east of the crash site #1. Don't perform the standard dive attack, instead try landing behind them and silently take them both down (Y) #2.
The two other sniper can be found on top of the roof, north-west of the crash site #1. It's worth to use the fact that they're standing beside each other and take them both down at the same time #2.
Now you can jump down and take a look around the crash site to find Vicki Vale #1. Have a short conversation with her #2 and afterwards you will be able to once again focus on finding Joker and retrieving the cure.