Possible Clues:

• Equatorial part of ocean with little prevailing wind
• Many vintage spirits cause depression
• Ocean regions near equator with little or no wind
• There's little movement in them, but there's mature spirit in them
• Detective sergeant initially imbibes vintage spirit for depression
• Tedious condition, still under canvas here
• Depressed state - area around the Equator
• Ocean regions with little or no wind
• Finally regained former spirits, in depression
• Windless zone