high-water mark

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A mark indicating the highest level a body of water reaches.
• noun The highest point, as of achievement; the apex.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A mark, such as a line of seaweed, showing the highest level reached by a body of water.
• noun by extension The peak or apex of something.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a line marking the highest level reached