The Bottlenose Dolphin is Tursiops truncatus, a highly social marine mammal that travels in pods through warm coastal Atlantic and Gulf waters. It is also one of the more intelligent animals in the ocean, known for its curiosity around boats and people. Dolphins use echolocation to hunt fish and often follow boats or ride their wake out of what looks like simple curiosity rather than any need to. Off a coastline like Leonida's, sighting one from a boat or the beach on a calm day would be the kind of thing anyone paying attention would notice. It appears in GTA 6 trailer footage, so Rockstar has confirmed it for the game. Whether dolphins react to a player's boat in any specific way has not been shown.
The community companion for Grand Theft Auto VI. Database, maps, guides and tools for Vice City and the state of Leonida.