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You must have completed the "Getting a Clue" main quest line to start this DLC (this is an early story mission). There are no specific level requirements.
Once you've done this main quest and downloaded the Far Harbor DLC, go to the northeastern corner of the map. There is a new area available called "Nakano Residence". Here you will meet parents looking for their missing daughter. Talk to them and get the daughter's last holotape from the safe in the boathouse. There's a key to the safe in the picture frame near the safe. Talk to the parents again and they will offer you their boat. Accept, climb aboard and you will reach the Far Harbor DLC area.
Doing this will also unlock the Far From Home trophy / achievement.

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