A lock is a water lift. The canal climbs a hill in steps. Each step is a pool; between two pools sits a lock: a chamber with a gate at each end.
Going up: a boat enters the low chamber, the gates close, water is let in from the upper pool (the chamber fills) and the boat rises. It sails out into the upper pool. Going down is the reverse: the chamber empties into the pool below.
Water is the scarce thing. Every fill takes a chamber-full out of the pool above; every emptying pours one into the pool below. A pool may not fall under its minimum, so the summit feeder stream (and boats coming down) supply the water for boats going up.
Keepers and hours. Nothing moves unless a keeper is standing at the lock, and keepers must walk between locks. Locks work only 06:00-22:00. A chamber must also be in the right state: an empty "turn" changes it without any boat.
The score. Every boat has a deadline and a priority (1-3). Weighted lateness = sum of priority × minutes late. Lower is better; water drawn breaks ties.