The Pillars of Hercules refer to the two promontories which border the Strait of Gibraltar. On the European side is the Rock of Gibraltar. The African side is in dispute, being a mountain in either Algeria or Morocco.
Before killing one of his fighters Giorgio tells him "don't forget to pay the ferryman". In Greek mythology when a person died their soul would be sent to the shores of the River Styx in the underworld. In order to reach the other side and enter Hades (the afterlife) they have to be ferried across the river by Charon, the ferryman. But in order to do so they must pay the toll for crossing the river, if they didn't have any money to pay Charon they would not be able to cross the river and enter the afterlife and their soul would be cursed to wander in limbo for eternity. Hence the ancient Greek custom of placing two coins over the eyes of a person when they are buried, the coins are to be used to pay Charon's toll.
The battery backup units used by the computer consoles in the CIC are CyberPower Backup Uninterruptible Power Supply (UPS) units. Backup UPS units contain a high performance battery and act as a backup power supply, whenever a power failure is detected the unit instantly switches to battery power, keeping connected devices running without interruption of service. They also are designed to protect against both power surges and under-voltage (brownout), a minor power surge is handled by routing the excess voltage into battery storage, a large power surge trips the circuit breaker switching the unit to battery power; if there is an under-voltage then the battery automatically discharges enough power to normalize voltage. They are primarily designed for use with computers to protect them against damage from power outages, especially the power surges/under-voltages that come with brief power outages. Having a computer connected to a UPS allows uninterrupted use during brief power outages (flashes), having internet modems and routers connected also prevents them from being rebooted by a power flash and prevents interruption of internet and network connection. A UPS also gives the user time to save data and properly shutdown devices in the event of an extended power outage, as most UPS units only have enough battery power to run a computer for 10-20 minutes, depending on the type of computer and other devices connected.
Davos rank insignia indicates he is a Vice Captain, which is the Hellenic Navy's equivalent to the U.S. Navy rank of Commander. Their officer's ranks are pretty similar to most navies: Ensign (O-1), 2nd Lieutenant (O-2), Lieutenant Commander (O-3), Commander (O-4), Vice Captain (O-5), Captain (O-6), Commodore (O-7), Rear Admiral (O-8), Vice Admiral (O-9) and Admiral (O-10).
"This way there be monsters" is an old saying among sailors referring to sailing into previously unknown/uncharted waters, specifically to the unknown dangers and hazards that entails. Old maps and charts of the ocean usually had "monsters" like giant octopus, sharks, sperm whale, giant squid and mermaids drawn in the margins or areas that had yet to be explored and charted to represent unknown dangers like navigation hazards and hostile life.