The Kraken Sub must be unlocked in GTA 5 before you can access it. It is unlocked by completing the 'Wildlife Photography Challenge' in single player mode. To complete the challenge, you need to take pictures (using the Snapmatic) of all 20 different kinds of animals that exist in GTA 5.
https://www.youtube.com/embed/V_P89tjnXPI
You can find a guide on completing the challenge here. At this time we only have the cell phone cheat for the Kraken, so you'll need to be playing on Xbox One, PS4 or PC in order to spawn it successfully (doesn't work for PS3 or Xbox 360).
