The correct answer is B) heights above water of land features such as lights. Mean high water is the reference plane used to measure the elevation or height of land features, such as lighthouses, navigational aids, and other structures along the coastline. This reference plane is commonly used for all vertical measurements related to these types of features, as it provides a consistent and standardized datum for referencing their position relative to the water level. The other answer choices are incorrect because: A) water depths are typically referenced to a different datum, such as mean lower low water; C) soundings are also referenced to different datums, depending on the region; and D) mean high water is not used for all vertical measurements, as there are other reference planes used for specific purposes.