Complex sentence: Oil slicks and air bubbles after the sixth and seventh attacks told "Griswold" that her quarry was hit--this was confirmed shortly before 0300 on 24 December when a periscope poked out of the water.
Split into simpler sentences: Oil slicks and air bubbles after the sixth and seventh attacks told "Griswold" that her quarry was hit. This was confirmed shortly before 0300 on 24 December, when a periscope poked up out of the water.