The sea chart enables the mariner to set his vessel in the direction he needs to achieve his destination, with a compass course when his current position, which will be changed by wind and tide, has been checked. It has to reflect varying information in a fluid situation, whereas a land map is the opposite - it records fixed positional information and the only variable is to record progress across it.
Early sea charts filled unknown spaces with artwork, giving credence to fantastic sea-monsters and non-existent islands, rather as the ill-informed raconteur fills out his story with description. "Here be dragons" is a phrase used to denote these dangerous or unexplored territories, in imitation of the medieval practice of putting sea serpents and other mythological creatures in blank areas of maps.
The necessity for, and availability of, more and more information on a chart, and the need for standardization, both as to production and interpretation, led to the chart becoming honed to a somewhat more severe style and serious execution. By the 1800s the fantastic was out, and the straightforward was in.