Possible Clues:

• Follow and observe for a two-hour period
• Two-hour period of duty on ship
• Follow Albert perhaps, on two-hour shift
• Two-hour period of duty on board ship
• Cerberus, say, exchanges sections for period of vigil
• 4-6pm or 6-8pm shift on a ship
• Night shift
• Duty period for ship's complement, kind of canine alert
• Late afternoon duty on a ship
• 1600 to 1800, on a boat