1. Literally, the highest level a body of water has reached, or the mark designating such a point. Due to climate change, the high-water mark on this beach has been getting higher every year.
2. The apex, peak, or highest point of something. The high-water mark of the country's economic boom was in 2007.
The peak of something, especially an achievement. For example, This composition is the high-water mark of his entire output. This expression alludes to the highest mark left on shore by the tide. [Mid-1800s]