Although seems like this is far north in longitude, the Crystalline Ruins is actually approached from the southern edge of the map, just south of the Huaka Sea fort located at (1231, 1420).
Go to approximately 1261, 1490 and anchor ship. Then walk south into the diamond offshoot. Diamond sentinels will be wandering in the wilds, blocking from fleeing but they can be calmed. All the way south is the entrance to the area. While moving south, the co-ordinates will "wrap" to the northern edge, starting from 0 and go towards 1261, 20.