The Energy Sea is a body of pure energy in the form of a sea in a system. All systems have energy seas, but some systems are built on top of their energy sea so it is not visible. The energy in the sea is extremely dangerous, deleting anyone unlucky enough to fall in. The energy from the sea is not usable until it is funnelled through the System's Operating Core. The Principal Office then distributes the energy to the rest of the system. Tears can form on Energy Seas easily because of the uncontrolled energy. Energy Seas are representations of computers' power supplies in the User world.
In Mainframe the energy sea surrounds the city. Mainframe maintains docks on the sea for ports to the Net. Some citizens have boats at the docks so that they can go sailing on the Energy Sea. Mr. Mitchell has a small sailing yacht at the docks in Beverly Hills sector.
As Mainframe's devastated sectors began to tear themselves apart, Bob was forced to go into the Core to power them down. All the while the system shutting down was causing the Energy Sea to spark erratically, until the sectors drew far too much energy at once for the city to control properly. The energy slammed into Mainframe like a tidal wave and the operating core funneled in the deadly energy inwards as sectors were off-lined. If it reached the Principal Office the entire system would be destroyed. Fortunately Bob managed to shut down the sectors and the energy's path halted, however the damage done to Mainframe was extensive. The sea turned dark as the city went offline. (System Crash)
When Mainframe began to crash sections of the city started to collapse into the Energy Sea. Bob suggested they let the city crash and hope the User would restart and restore the system. Except for the Principal Office the entire city collapsed into the Energy Sea. The Energy Sea returned to normal when the User restarted Mainframe. As the system restarted, the energy resumed flowing from the energy sea into the city once again. (End Prog)
Mainframe's Twin City shared the very same Energy Sea. Both cities funneled in their energy which overlapped each other. After Welman's experiment went awry, its Principal Office exploded and the entire system was destroyed. All that remained was a chaotic ruin, thus the energy ceased to flow altogether. (Daemon Rising)
In the Super Computer the system is built over the energy sea with bridges connecting every section of the system.
In the Desert Port System the energy sea is more like a lake. The city and coast surround the sea, there is a circular port in the center of the sea allowing access to the Net. (The Episode With No Name)