I spend quite a bit of time in small fishing boats and canoes, and have tried a variety of different bags. I currently use a soft bag that I bought at a fishing store designed to keep a six-pack of beer cool -- it's waterproofed nylon on the outside, has insulation which serves as padding, and has a full plastic liner. There are three pockets arouind the edges for small accessories. While it might not be technically waterproof, the inside plastic liner is seamless, and no water has ever gotten in. But -- it's not a backpack style. It looks like a squarish old-fashioned camera bag with a shoulder strap. I put a Domke liner in it to separate lenses from body. In terms of size, it's big enough to hold a six-pack plus one of those plastic "blue ice" things that you freeze and then use for cooling...so say about half again the size of a six-pack. Mine was made by Rapala, the fishing gear maker, and I bought it on-sale for about US $20. The store had quite a variety of the things, in different shapes and sizes; I don't carry my Nikon gear in it, but I believe it would hold a D2x body and as many as four lenses, if they were smaller.
Take a look -- you can find them in any well-stocked fishing store, assuming that Australians drink beer.