Florida has a lot of water. So does Leonida. Beyond the obvious coastline and the Keys, there are inland rivers, tidal channels, urban canals, and wetland sheet flow.
For the urban side, see the Vice City canals piece.
Water types visible across the map
Coastal water - around Vice City, the Keys and the offshore islands. Tidal estuaries - where rivers meet the sea. Inland rivers - draining the higher ground (such as it is) into the wetlands. Urban canals - inside Vice City. Sheet-flow wetlands - the Everglades-coded centre of the state.
Each behaves differently. Coastal water is high-traffic; canals are slow-traffic; wetlands are barely-trafficable; rivers are somewhere between.
Why this matters
Boats and aquatic vehicles connect different parts of the map in different ways. The bridge route is one option to the Keys; boats are another. Wetland traversal is mostly airboat or small boat. Coastal cruising is yacht or jet-ski.
Water is also part of how the map breathes. Not every drive is on a road, and the alternative routes give the map a non-linear feel.