After a lot of trial and error (mostly errors, to be fair) with the "classic" way of unlocking this achievement, I found a much safer and faster solution, exploiting the stolen crown mechanic on decay.
Fast cheesy solution
1- Create a new save, build the boat and sail to island 2. Once there, either let your crown get stolen, or return to island 1.
2- On island 1, get your crown stolen 5 times. Each time it happens, it adds 100 day of decay to all the other previously visited islands. In our case, island 2 is now concerned. So the 5th time, island 2 should have 500 days of decay.
3-rebuild your boat and sail to island 2 again, and enjoy your achievement. I know the achievement description says 200 days, but decay happens slower in islands 1 and 2 so I want to be on the safe side.
Edit: CheshireMulisha mentioned that he obtained the achievement only after building a wall.
Classic solution (not recommended)
You want to come back to an island that you previously visited, which suffered decay for more than 200 days. The problem here is that the decay mechanic is voluntarily left unclear by the developpers, and many elements interefere with this achievement. For instance, decay happen slower on island 1 and 2, so you need to wait longer if you want to unlock this achievement by returning to one of those islands. You also need to factor walls (they prevent decay for a short period of time), and possibly lighthouse. As you can imagine, you need to wait a lot, but you don't know for how long.
By the way, the devs confirmed to me that walls impact this achievement, and that each iron wall reduce decay by 30 days. I don't have the values for the previous levels of wall.