Comments from my experiences of rafting thru the Grand Canyon...
* Using a Pelican case I was thought I was being very clever when I decided to cool the case off by emersing it in the water, (my film was inside the case too, it was very hot down there and my gear was on top in the sun). To my surprise I saw bubbles rising up from the case. Upon inspection, the case had water in it. Reason? The gasket was about three years old AND my Pelican cases always had the lids closed since I used them as my regular cases. This wears out the gaskets faster. Put new one in before you go and take a spare. Keep the gasket free of sand, etc., and take a jar of petroleum jelly to treat the gasket after cleaning (if necessary).
* Depending upon your situation and where they store your gear try to have it on the bottom of the center heap to keep it out of the sun. If it is a very large raft it may be possible to store in in their under the deck storage space midships. This will keep things out of the sun.
* Do not set up your tent at the water's edge, no matter how romantic this might seem. At night the water level can rise. The Dam upstream adjusts how much water it releases. I found it very difficult to find another camp site in the middle of the night.
* Although I knew some about geology and the guides explained things, had I had boned up a bit more it would have been much more interesting and enjoyable (although maybe not directly related to making images). Going down river the geology gets older and older until it is the oldest stuff on the planet?
Enjoy and remember to hold your breath when you are under water!