1. A marine navigation device comprising: a location determining element configured to determine a current geolocation of a marine vessel on a first body of water; a display; a memory element configured to store sonar data for substantially all of each of a plurality of bodies of water; and a processing element in communication with the memory element and the display, the processing element configured to— access the sonar data from the memory element corresponding to the first body of water, calculate bottom hardness shading data based on the sonar data, generate a map representation of at least a portion of the first body of water corresponding to the current geolocation, the map representation including shading derived from the calculated bottom hardness shading data, and control the display to visually present the map representation; wherein the sonar data includes a plurality of geolocations for each body of water such that the geolocations include all geolocations of each body of water that are navigable by the marine vessel, each geolocation being associated with a depth of water value and with a hardness value which varies according to a density of the soil and other underwater terrain in the geolocation; and wherein the bottom hardness shading data includes a monochromatic shade value of a first color for each geolocation of the sonar data, such that each monochromatic shade value varies according to the hardness value of the associated geolocation.