the highest point that the water touches
No, there is no hyphen in "high water mark." It is typically written as three separate words. However, if used as an adjective before a noun, it can be hyphenated as "high-water mark."
High Water Mark of the Rebellion Monument was created in 1892.
the state owns the land below the high water mark
What is the famous battle in Pennsylvania became known as High Water Mark of the confederacy
The Battle of Gettysburg is often referred to as 'The High Water Mark of the Confederacy'.
If by "high water mark" you're referring to flooding, there really is no way to calculate it. howver, if the question is how to determine of a property is subject to flooding, the US Department of Housing and Urban Development has maps which will show lands which are located in a 50-year or 100-year floodplain.
Gettysburg
Gettysburg
Gettysburg is sometime called the "high water mark of the Confederacy."
Mark
Generally, private property ends at the mean high water mark. However, you may need to check the laws of your particular state. The laws in Florida are being modified where the state has added footage to the beaches for erosion control projects. The state does not consider that added land, which changed the old mean high water mark, as belonging to the abutter and suits have been filed. The added beachfront has been dedicated to the public.
During a spring tide, the high water mark would be high up on the beach on the flood, and very far out on an ebb tide. During a neap tide, the high water mark is not so high or so low as during a spring tide.