Contrary to popular rumor, there is no hidden code that makes a tsunami hit Los Santos in Grand Theft Auto V. However, an unofficial fan mod allows you to explore a partially submerged Los Santos in a submarine.
The "Tsunami Mod" by Bushigan was released in early 2014. Rather than causing a tsunami to hit the city, the mod changes the game map to reflect how Los Santos might look in the wake of a tsunami. Players are given a small submersible vehicle with which to explore the underwater portions of the city. This mod is not made or supported by Rockstar.