There are no such thing as plaster cracks!
This is a very common pool crack. It is not a "plaster crack". It is caused by the mason who installed the tile on this pool not removing all of the duct tape used during the gunite installation back when the pool was first built. Tape is put over the skimmer to keep the gunite out during the application.
G&B Tile and Plaster 960 North Mill Street Lewisville, Texas 75057 Phone (972) 906-0142 www.poolsub.com