Pull the Plug

Pull the Plug is a side quest which occurs at the Thicket Excavations in Fallout 4.

Detailed walkthrough
Sully Mathis wants to unblock some underwater pipes at the Thicket Excavations. The pipes can be located by finding the bubbles on the surface and swimming down (water is irradiated, 10 per second). Each pipe can be fixed by simply turning a red valve located on the pipes. There are two exits from inside the dam, there is a lift and a stair case which are on opposite sides of the dam. The lift has a button at the top and bottom, having it in the right position prior to navigating can be helpful. Having the perk Aquaboy or the female equivalent can help with navigating in the water due to its water breathing effect. Using a mirelurk cake can also help as it provides water breathing if the Sole Survivor does not have the perk, and some Rad-X works to counter the high rads.

After the player character fixes the pipes he can leave the zone and come back later or simply sleep 24 hours in the trailer right behind the big pump to find out that the lake has been drained and occupied by raiders. At the very bottom of the dig site the player character will find Sully (now a much tougher boss, having a minimum level of 30) and several mirelurks in pens which can easily be killed if the player character doesn't open doors leading into their enclosures. In many cases, a single frag grenade should suffice. Sully seems to be breeding the mirelurks, for unknown reasons.