Mega Tsunami, unlike a traditional Tsunami, is a rare and destructive form of Tsunami. The waves in such a Tsunami are several hundred meters long, capable of drowning mega cities and provinces. Such a Tsunami has not occurred in previous centuries but is predicted to come across the East Coast of America, North Africa and Europe. Since Vice City is based on Miami which has low risks of being hit by a Mega Tsunami, players have claimed it to occur in GTA Vice City.
The origin of the Mega Tsunami myth dates back to the video series by MrRiley365. The user created a scenario in which Tommy Vercetti was drowned during a Mega Tsunami in Vice City, turning the entire city into an ocean. The first video is followed by a second narration of the event described by a character called Agent Sean, this video also includes Ken Rosenberg.
However, the video is just a predicted scenario of the event and doesn't occur in the game. The only possible evidence of the city being flooded is the ocean glitch in the Marina Parking Lot. Rare glitches might cause the ocean water to overflow in certain parts of the city but this is only possible if the game files are altered with, making this myth false.