Poseidon Energy is a power plant controlled by the gunners. Here, you find quite a few valuable items, but you will also have to defeat many strong opponents. The location has been divided into two smaller ones - the flooded basement infested with mirelurks and the main part of the building occupied by gunners.
It is impossible to get into the building the regular way, because all of the doors are locked from the inside. You need to approach the building from the waterside, where there is the entrance into the building through one of the pipes.
The entire bottom level is flooded by water, which makes it more difficult to move around there. Additionally, you encounter lots of glowing mirelurks here. You can move between individual rooms along underwater tunnels.
As soon as you exit the basement, you find door that you can open with the novice level terminal. Across the door, you get to the main part of the building. Here, you encounter a dozen-or-so gunners, several roots and automatic turrets.
You should especially watch out for the gunner leader - Cutty, who is staying in the room at the upper level. He is wearing a power armor and is in the possession of a key that you can open all exits with (which are locked with master locks).
In the same room, above, on the desk, you find the Endurance Bobblehead and an issue of Tesla Science.