The skill is the ability of the character to read the waters and its ways. The extent of the information the character will know depends on how high his/her BCS in the skill is.
Example: A character can tell that their vessel is coming up to a shallow reef by the way the water is breaking. Or being able to tell which direction the current is flowing and how fast. With the Ocean Seamanship skill the character will be able to tell if the tide is coming in or going out. Etc.