I don't have an Icon brand, and you may already know the following, but:
I don't think you'll find a tank bag that is truly waterproof rather than water resistant. Evan a rain cover is just a stop-gap to help keep the contents of the bag dry.
The denernier of the weave and a water resistant coating can help, but the tank bag has zippers, and even if they are covered with a flap, they will will still leak when the rain is hard and you are going fast enough to drive the water in.
So I think you will want a rain cover. I have the RoadGear products, which have rain covers (that you buy separately). Their stuff is expensive, so you might want to either return your Icon and get one that comes with a rain cover, or order a rain cover for your Icon from RoadGear.
Good luck, and let us know what you decide.