Pull the Plug

This is given by the NPC named Sully Mathis in Thicket Excavations if you agreed to help him get the water pump working. During the first time conversation with him, you'll have speech checks allowing you to ask for more money and ask what he's really up to.

After looting the nearby trailer cars, head to the flooded basin and dive to the spots where there are bubbles coming up from the Water. The water's radioactive so make sure to use Rad-X to minimize the radiation inflicted. You need to fix three pipes so remember to get out of the water and take a breather. It will be better to circle around in the surface and dive to the spots near the bubbles.

After repairing the pipes, talk to Sully then activate the circuit breaker beside the Water pump. Shortly after, mirelurks will appear. Kill them both then talk to Sully again to complete the quest.