You find it, more or less, in the "treehouse area." However...

You don't see it while walking around up in the treehouses.

You don't see it while standing on the ground (i.e., while standing on dirt).

You see it while walking on the walkways that are at water level.

You might have to position something a certain way before you can see all of it.

Even if you haven't "positioned something a certain way," you can still see half of it -- specifically the ending half.

Walk around on the walkways at water level, and you're looking for the curved end of a line.

Once you see the end of the path, you'll probably figure out how to arrange it so you can see the beginning.

When you see it, you'll be standing right where the boat docks.

While standing near where the boat docks, look at the exact spot where the boat docks.

If you can't figure it out by looking at the end of the path, then summon the boat to the dock.

The boat has to be facing a certain direction to see it. If you need to turn the boat around, sail it 100 feet or so, then turn around and come back.

Drawing the path requires a little bit of waiting.

The starting circle is created from a reflection of the front of the boat in the water. The line then travels up the buoy. Wait for the buoy to rotate in order to complete the path.

The solution involves two objects that move, only one of which you can control.

It's not the circle on the back of the boat.