Grand Theft Auto: Vice City
The Yacht is owned by Juan Cortez, and is featured in all the missions Tommy Vercetti does for him. In the mission All Hands On Deck, Tommy must defend the boat from the attacking GIGN and DGSE agents. You can enter the Yacht using a helicopter: land it next to the boat and then jump aboard it.
The Ballad of Gay Tony
The yacht appearing in TBoGT is owned by criminal Frickie Van Hardenburg. During the events of the game Luis Fernando Lopez is ordered by Yusuf Amir to steal the Buzzard helicopter off the boat and destroy it. This time, the Yacht is larger and has a helipad. Unlike in GTA: Vice City, you can now enter it.
The boat can be driven with the use of mods, and it makes a cameo in GTA V in the movie Meltdown.
Grand Theft Auto V/Online
Also in GTA Online after the Executives and Other Criminals update (enhanced version only) is the Galaxy Super Yacht, which can be purchased by the GTA Online Protagonist, functions as a safehouse, and comes with several other vehicles.