In Dartmoor Garden Show, it is found on the gardener's table outside the greenhouse, sitting next to a potted plant. You can pick it up without anyone suspecting Agent 47 even though the key icon has an exclamation mark on it. You should also check if the executioners are nearby.
You should know that the exit cannot be unlocked forever, as Agent 47 is visiting the location for the first time in each part of the mission.
After presenting you this guide on where the boat key is in Hitman 3 Dartmoor Garden Show, we hope you can use the key to climb up to the boat exit in the mission.
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