Grassrivers
The Everglades analogue. Wetland, airboats, alligators, and one of the largest natural regions on the map.
Grassrivers is Rockstar's Everglades: shallow water, sawgrass, airboats and wildlife that can kill you. It is one of the biggest natural regions in Leonida.
It also produced the most striking image in the official screenshot set, with Jason and Lucia at sunset as three police airboats and a helicopter close in on them.
The name is the geography
Grassrivers is close to a direct translation of what the Everglades actually are. The region is not a swamp in the usual sense but an extremely wide, extremely shallow river of fresh water moving slowly south through sawgrass. The Seminole name usually given as Pa-hay-okee translates as grassy water, and the phrase river of grass entered common use through Marjory Stoneman Douglas's 1947 book of that title.
Rockstar naming its region Grassrivers is a reasonably straight reference rather than a joke, which is unusual for this series and suggests the area is being played with some sincerity.
Why it is good level design
Open water with no roads is a difficult space to make interesting and the Everglades solves it. The sawgrass is tall enough to break sight lines while the water stays navigable, so it is simultaneously open and enclosed. Airboats do not care about depth, which means pursuit can go anywhere. There are no natural chokepoints, so an escape is about direction rather than route.
Grand Theft Auto has never had terrain that behaves like this. The closest previous equivalent is the Blaine County backcountry in Grand Theft Auto V, which was still fundamentally roads and hills.
The wildlife
Alligators are the obvious presence and the screenshots include other Leonida wildlife. Red Dead Redemption 2 built a great deal of its world out of animal behaviour, and the studio bringing that into a modern-day setting is one of the more interesting unknowns about this region.
What is not known
How large Grassrivers is relative to the rest of the map has not been stated, and Rockstar has not confirmed hunting, fishing or any survival systems. Airboats are shown in police hands. Whether the player gets one has not been demonstrated.