I believe that the base maps is a road map for the US, but no kind of water, or topo data is in the device.
There is NO base map in the Voyager. When you turn it on the first time, the map screen is indeed empty. You cannot load Garmin-style maps into the Voyager. The only thing you can load are files that contain LINES. These lines may represent streets, roads, trails, tracks. You make the file containing lines using other resources, like openstreetmaps - converting streets to lines and then load them into the Voyager. In use, tracks you make while riding are shown as black lines.
You can download the Voyager User manual from the TrailTech web site.
You will be suprised at how limited the unit really is.
OTOH, YOU control what is on the display and for some, that is all that they require. Being able to make maps (as gray lines) is all that some riders want - they see all the other info as clutter.