Factual error: 1. Why wasn't Prairie/Masker operating all the time they knew a sub was out there? That's SOP for the US Navy. 2. Spearfish torpedoes are homing torpedoes. They wouldn't just shoot by on each side of the ship, but if they did, they'd turn around and pursue their target until they ran out of fuel. 3. Why didn't the ship employ countermeasures when the sub launched torpedoes? A noisemaker, a nixie - Navy ships don't just cross their fingers and hope the torpedoes miss.
Factual error: Sean says he was the coxswain on HMS Achilles. However, in the previous episode's flashback to the beginning of the red flu outbreak when Achilles was still in operation he was shown wearing lieutenant's rank insignia. The coxswain is the senior rating aboard a Royal Navy submarine and holds the rank of chief petty officer or warrant officer; he is certainly not a commissioned officer.