After getting over my initial reaction of "Holy **** it's Garrus Vakarian! I wonder if he needs anything calibrated?", I took a look around the flooded structure and came across Sully's computer. Some of his journal entries lead me to suspect that he's a slaver or a raider (something about traders being "easy pickings" and considering making somebody else try to fix the pipes), in which case I definitely don't want to help him. However, I went back to talk to him again and did not see an option to confront him about what was on the computer. Did I miss something, or do I actually have to fix the pipes to open up any new dialogue or plot points on this quest?

I did the quest... then put a bullet in his head... or 5...

SPOILER

I helped him, read the entry, waited for days, but nothing happens. The whole place is supposed to turnto a raider base, with Sully as the leader. The quest is shown as finished, despite still saying "talk to Sully":

No worries, do the quest for Sully and eventually, the quary indeed turns into a raider base. Just do something else for a bit and then return. Sully will be waiting with his friends and l33t lootz!

Think of it like this...

You are planting the seeds to harvest a crop of loot from the blood fields.

I have the same issue, the quest shows completed, but the check mark by "talk to sully" is not there, like the quest is not done.

Any ideas?

I did a lot of more side quests waiting, and I don't want to have to do those again.

PS. Glad I wasn't the only one this happened too.

After some time, it finally happend. But the fight wasn′t easy because of the graphics glitch. Those buggers shot me, but i couldn′t return the favor, atleast not from afar.

Part II of the Thicket Mine is only doable after the charter is at least 12th level. And it's not really a quest because you aren't doing anything for anyone specific. It's just you dealing with Raiders, which happens all over the map.