You can find a room with many things after using the zip line with a hook.



Remove all furniture through the window.
If you drop it into water, it will respawn in the room so be careful.

You can find a torch after the part of the rolling barrels.



Go up spiral stairs and set fire to wood.

You can reach the island by boat or ship in the picture below.


The hidden island is located in the opposite position on this map.
(This guide shows a clockwise route but it is faster to take an anticlockwise one.)
Ignore the island of the goal and go along the rock wall on your right side.


Keep going ahead and you can find a small island far away.

Land there and unlock the achievement.

Just complete the level.

At first, immediately after the Port update, the achievement "To beach their own" was removed while I was playing and seeking a way to complete it.
However, this is back with the later update "Under Water".