The correct answer is A) to indicate the shoreline where there is a large tidal fluctuation. Mean high water is the average of all the high water heights observed over a 19-year period. It is used as the reference line for shoreline delineation in areas with significant tidal ranges, as it represents the typical highest reach of the tide. This is important for mapping the shoreline and defining property boundaries. The other answer choices are incorrect because B) is for rivers and lakes, C) is for bottom contours, and D) is specifically for the Gulf coast, while the question is asking about the general use of mean high water.