The Sewer.esp Mod
The Sewer.esp will allow you to access the Sewer level. To get the mod to work on your copy of Fallout 4 please download the Sewer.esp Here.
Once the Sewer.esp has downloaded, find your data folder for Fallout 4 on your disc drives under SteamLibrary>steamapps>common>Fallout 4> Data. Then take the Sewer.esp and copy it into the data folder.
Once the Sewer.esp is installed, open Fallout 4, go to Mods, sign in or create a Bethesda account, click Load Order, and check the Sewer.esp in the list.
If there are other mods enabled, please disable them as they might interfere with the Sewer.esp mod.
Once the mod is installed go to Sanctuary in game. While the actual mod was made to be based in Lexington, I have dropped the Sewer's entrance under Sanctuary's destroyed bridge for easy access from the beginning of the game. The entrance will be a drain pipe sticking out of the water as shown in the above picture. It will be called the On Site Water Lab in game.
A few tips to explore or test the mod: use ` (tilde) in game to bring up console commands. This will allow you to use two very helpful commands, TCL (toggle clipping) and TGM (toggle god mode). These commands will allow you to move through walls and bypass locked doors (TCL) and become invulnerable (TGM), avoiding all fatal threats in the Sewer. If you want to play the mod naturally, I would recommend a level 15 character or above.
Thank you for checking out the mod and I hope you enjoy it.