I see. I think it would be hard to nail into the sides of the deck boards if they are level with the pool so you might have to use small blocks that you first fasten to the finish piece then fasten the block to the deck underneath when the finish piece is in place. You also would probably have to remove these pieces every year to put the winter cover on and re-install them every spring.

One thought - they sell white rubber material for the sides of garage doors to cover the gap between the door and the wall to keep rain snow out etc. I wonder if this would be wide enough (and look good enough) to bridge your gap. Since it's rubber, you could probably still get your cover on and off with them in place. It might be worth a look next time you're at a hardware store.

Peter