ADC is short for Acoustic Device Countermeasure, and it is used to deflect torpedos directed at a ship.
According to the DVD extra, Executive Producer Hank Steinberg stated that the 12 'o clock reports, where the orderly asks the Captain's permission on behalf of the Officer of the Deck " to strike eight bells on time", was implemented to keep mutiny in check. Eight bells signify the end of a four hour watch, with each bell marking the passage of an half hour of time.
In this series the world in terms of technological development essentially ended in mid 2014 with the outbreak of the global pandemic. The most common version of Bluetooth at the time was Bluetooth 4.1, which was released in December of 2013, it had a data transfer rate of 24 MB/s and a maximum range of 300 feet, v4.1 is what most phones would of had as of mid 2014 when the pandemic hit. Bluetooth version 4.2 came out on December 2, 2014, it has the same data transfer rate and range as 4.1 but was faster in that it used larger data packet sizes and also was the first Bluetooth version to have an Internet Protocol function, meaning it could be used to connect to the internet. While the maximum stated range is 300 feet (outdoor line of sight range) (Lt. Granderson says 90 yards) its practical indoor range is more like 50-60 feet in average conditions. The next major update in Bluetooth technology came in June of 2016 with the release of Bluetooth 5.0, which has substantial improvements over the previous version; it has double the bandwidth at 48 MB/s and over triple the range, with its maximum outdoor line of sight range being around 985 feet and an average indoor range of around 130 feet.
When the Achilles fires torpedoes on the Nathan James one of the weapons control officers recommends using a streaming nixie. He is referring to the AN/SLQ-25 Nixie, which is a torpedo shaped device that is towed behind a ship as a decoy, the Nixie has a propeller designed to make a lot of noise, which then fools the torpedo's passive sonar into thinking it is following the sound of the target ship's propeller when in fact it is chasing the decoy.
A cellphone with Bluetooth 4.1 would have a maximum outdoor range of 90 yards (270 feet) as stated by Lt. Granderson. Sean is using these phones to send messages across the state of Florida, which Cap. Chandler says is roughly 300 miles. Which means it would take approximately 5,867 people/phones to send a message over a distance of 300 miles using Bluetooth.