Yeah, it was simple. It was that checkpoint that's inside the tunnel going to the keep that kills me. If you swim in the water above the tunnel you can see the green beam of light which is actually the marker in the tunnel, so I kept trying to go for it in the water. I didn't even bother going into the tunnel because I kept thinking, 'It's a rooftop race, there would be no reason for me to go into this tunnel." Well, I was wrong.
Woo to completing all rooftop races.