"Use the Trackify app to locate the device" is an objective in the story mission The Merryweather Heist (Off-shore) in Grand Theft Auto V.
Walkthrough[edit]
Once Trevor is in the water and has launched the Trackify app you need to find the container. Look at it. Spin to face it immediately. From there just push forward near the sea bottom. It can take a minute to get there but more often than not you will nearby it. Just drive the Submersible over it and Trevor will pry it loose.
Once you have it just ascend straight up. There is little to worry about during this time. Once you do, switch over to Michael.
