Florida has lots of water. So does Leonida. Not just the coastline, not just the Keys, but inland rivers and tidal creeks, man-made canals in urban areas, and sheet flow through wetlands.
See the Vice City canals piece for the city's waterways.
Water types visible across the map
Coastal water - off Vice City, the Keys and offshore islands. Tidal estuaries - where rivers flow into the sea. Inland rivers - draining the hills (such as they exist) into the wetlands. Urban canals - located within Vice City itself. Sheet-flow wetlands - the Everglades-coded central portion of the state.
Each behaves differently. Coastal water handles high traffic. Canals are slow traffic. Wetlands are near impossible to get through. Rivers fall in the middle.
Why this matters
Boats and watercraft link map sections together in various ways. The bridge route is one way to the Keys; boats are another. Crossing the wetlands is primarily done via airboat or dinghy. Cruising along the coast means a yacht or jet-ski.
Water is another element of how the map breathes. Not every route is on a road, and those alternatives are what make the map feel non-linear.