In Thread Validator, Watermarks are event markers in the allocation history at which you can say other events occurred either before or after the watermark.
Watermarks are used in the Active Objects and Analysis tabs as a kind of filter to constrain displayed data to that which happened between two watermarks.
You can add new watermarks directly from allocation items displayed in the Active Objects and Analysis tabs, using the popup menu options.
Watermarks added this way are initially named using the .exe or DLL and the function name as in the picture below, but you can change this if you want.
Alternatively, you can add a watermark at the most recent recorded allocation event:
 Managers menu 
 choose Add watermark at most recent trace  
 enter a name 
 click OK 
Or use the option on the Watermarks Toolbar:

 The most recent trace event may not actually be visible in any of the displays as it could be filtered or hidden for other reasons.
There are two permanent special watermarks, not directly associated with particular events:
•The first watermark is the point before every other event 
 
•The last watermark is the point after every other event
These two watermarks are the default settings, meaning that no data is filtered due to watermarks.
You cannot remove or rename the first and last watermarks.
When a session is active, you can show the watermark manager to see a list of watermarks, change their names, or apply selected watermarks to the data views:
 Managers menu 
 Watermark Manager...  
 shows the Watermarks dialog 
Or use the Watermarks Toolbar option:

Watermarks are shown in order of their associated event allocation history.
If you haven't added any watermarks yet, this will just be the special first and last watermarks.

 The watermarks dialog can only be shown when the Active Objects or Analysis tab is open.
There's a few options for renaming and removing watermarks:
•Edit... 
 rename the selected watermark 
 
Double clicking on the watermark also works.
You can't edit the first and last watermarks.
•Delete 
 delete the selected watermark
 
You can't remove the first and last watermarks.
•Delete All 
 delete all the watermarks except the first and last
You can't change watermark locations. If you want to do that, delete the watermarks you don't want and add new ones.
•First 
 set the earlier of the watermark range 
•Second 
 set the later of the watermark range
You can't choose a second watermark which is earlier than (or the same as) the first one.