You need to be on the boat.
Map
Treehouse Obelisk
North 2
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You need to be moving.
You need to be moving clockwise.
The puzzle is dark green.
It starts near the boathouse dock, then goes around the shipwreck and ends near the treehouse dock.
Complete spoiler:
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
The puzzle is on the side of the shipwreck.
Show hints
Intro
This one is very fun once you find it.
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE