I recommend the watersports drybags manufactured in a variety of sizes by Cascade designs, available at R.E.I. or the like. Get the smallest size that holds your stuff.
If the back is not completely filled, it is difficult to close it in a waterproof way. If your back is too big, take a partially inflated beach ball to fill it up.
Some people prefer a ordinary day pack (which will get soaked), containing one or more smaller drybags. A disadvantage of this method is that you are carrying heavy quantities of water. This holds especially if in your day pack there is any room left. Fill empty spaces with inflatables.
I prefer one drybag containing all my stuff. The smallest of the bigger sizes (i.e. with backstraps) is only a bit too big. When I carry a camera that cannot swim, I take a small drybag inside the bigger one. As closing the bags takes some time, it is a bit of a hassle to take pictures this way.
Those who go for maximum security may choose to pack their sleeping bag in a small drybag inside the main drybag (this method is too heavy for my taste). Also, one might want to take rubber cement and/or a patch kit for dry pack repairs along the way, in the extremely rare case of a puncture.