In the "Onboard Computer" prep mission for Act 3 of the Doomsday Heist you'll need to intercept and shoot down a prototype Hydra, then dive underwater to recover its flight computer module.
You need to shoot down a prototype Hydra in this mission and need to get either a Buzzard, which is easily accessible through CEO Vehicles in the interaction menu, or you can use an Oppressor Mark II if you have one. We also recommend that you buy a couple of rebreathers, as you will need to get the module by diving underwater after shooting down the Hydra.
Get into either your Buzzard or Oppressor and head towards the Hydra. When you get closer to the Hydra, you wont be able to lock on to it and will need to manually get closer and shoot it down. Two missile shots should do the trick.
Once you've shot it down twice the Hydra will crash underwater. Go to your interaction menu and put on rebreathers from the gears section. After that, head underwater and recover the module from the cockpit of the crashed Hydra. Once you recover the module, deliver it back to the facility.
