Well, I am running back to Tiber Island, I accidentally jump in the water and BOOM I fall for about 10-15 seconds and then I am back in invisible water!?  I look up and I am literally under the entire map of Rome...I can even make out most of the distinguishable landmarks!  Well, I let the timer run out on my mission so I could reset....did it again.  I try a third time, not going in the water, and the mission finishes perfectly.  Outside of the mission, I jump in water again to test and it happens a third time!  By that point, I just cut my game off and I haven't turned it back on (last night).