The Leonida Keys Safehouse is out on Key Lento in the Leonida Keys, the island chain south of the mainland. It is a confirmed story safehouse, giving the characters a base away from the city. How players end up owning it has not been confirmed, so hold off on assuming a purchase system.
Head to Key Lento in the Leonida Keys to reach this safehouse once the story opens up the islands. It is a confirmed campaign location, so access likely comes through the plot. Rockstar has not explained property purchases, so treat the way you gain it as unconfirmed.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.