When a Water item or multiple Water items are selected, the SELECTION - TOOLS menu becomes available. The SELECTION - TOOLS menu for Water contains tools that allow you to adjust your Water Level and create Boundary or Polyline items at terrain intersections that can be used for further volume analysis.

Water Group

ClipClips the Water item's extent to a selected Boundary. Select both the Water Item and desired Boundary to enable the Clip tool.
Water LevelDisplays the Water Item's current level and allows adjustment of the level through the arrows, or by entering the desired level.
Intersect TerrainCreates a Polyline or Boundary item where the water intersects with the terrain.  This is a Peak feature only.

Result: Boundary or Polyline