I'd advise unlocking this in conjunction with the 'Mariner' achievement as it will take up some time travelling to each of the boundaries. If you really want to eat up time to unlock the 'Mariner' achievement then travel in one direction until you hit an invisible wall and then just keep sailing whilst hugging the wall until you eventually do a complete circle (well square) of the map, hence hitting each of the N/E/S/W boundaries. Or conversely, if you want it popped quickly, just head either N/E/S/W until you hit the invisible wall and repeat for every direction. Use 'X' to open up your map to ensure that you've hit each boundary