I remember going for walks with my grandad and his whippet, we would go on this large open park and it used to tease the hell out of all the other big dogs, boxers, alsations, dobermans, but they could never catch it!
Looks waterproof to me, I would probably add a lead strip about a metre long and 9" wide , to protect the joint between the fibre glass and those 3 tiles, apart from that it looks neat.
Well, looking how the original fitters have left the window, you have to wonder if they bothered sealing both ends when attaching the stub cill to the frame.