Here is my 100% Achievement guide for Rainswept!
Another story related one, but there a few things to do before we get going.
First off, Search the car and take all items (Scissors and fuel container).
Then, fill the car with petrol.
Go left to the boat and cut the rope.
Grab the rope, then attach it to the car.
Go back to the left to the boat and attach rope onto that too.
Go back to the car and press drive.
Get the fuel out of the car, put it in the boat, then drive the boat, achievement unlocks.
Go to 1:01:40 to see where this unlocks
Hope this helps!