Reverse and crash the nearby truck into the building with the safe. Maneuver the safe into the truck after crashing into it. Drive into the water on your right.
Then go to the last cabin and crash into it with the two boats nearby. Aim for the supports that go into the water, this should collapse the entire cabin into the water. If any part of the cabin is sticking up, just ram another boat into it to flatten it.
You don't need the optional safe to trigger the achievement.
Good Luck.