You'll now have to bring the water pump from the workshop north of Marik's Marina. To reach the workshop, head to the northern side and follow the broken pathway around and climb up the ladder. Kill the enemies below. Clear the interior workshop then open the door. Grab the pump and place it on the boat outside.
Return to the mission building and bring the pump to Trevor.
@Guest I ran into the same thing and ended up going through the mission about six or seven times prior to getting it to work properly. I watched videos on YOU TUBE to see how other completed the mission. Each time they went into the area with the pump, killed the creatures inside of the fence and building and the exited through the double doors. I never did this. What I did each time defeat the creatures in the building and then I would throw the pump over the fence, climb onto the roof, jump down and retrieve the pump and take it back to the mission. I noticed that even though the map showed I had the pump, it also showed the pump still in the building (on the map only) and would not trigger the quest completion when I returned it to Trevor. If you open the double doors to the building where you found the pump, it seems to trigger that you retrieved it and allows the quest to progress whether you exit via the doors or the roof like I did.
I brought the pump to trevor, but he says go get the pump? I don't understand. Can anyone help? tks