I got as far as pulling the plug (fixing the three pumps). I then went to speak to Sully Mathis who was by the controls. He asked me to press the button at which then brings about a swarm of mirelurks. I kill them and he stays by pump controls and tells me he is busy or something. I cannot engage further with him, but I have permanent quest marker on the location, only with an empty square and not solid. I have since been back to wipe out a load of raiders as part of a BoS mission. In the bottom of the quarry was pens of Mirelurks so I wiped those out also. I'm still stuck with the quest marker constantly present.