You have loads of options.
You could just use an aquapac as sugested above.
Another option would be to wear a camelbak or other hydration pack and put your bits in a small waterproof pack inside that with the added bonus of having a portable water supply.
Another option, though not cheap, you can get little packs that attach to the back of most harnesses such as this.
Then get a small aquapac to put your dry stuff in and place this in the harness pack.