The correct question (as you point out) is WHO was on the water tower?
The person on the water tower was a professional assassin.
Their job was to take out one of the counter sniper teams.
They failed—and the counter sniper team took them out instead.
Obviously that meant there was a conspiracy—which is why the Secret Service, Homeland Security and the FBI have no intention of releasing any information on that.
Citizen investigators are going to have to do the work.
Nobody was on the tower - the Dome thing at the top not big enough for a shooter to hide behind either.