What to Bring
The limited space in a raft makes careful planning essential. A camera with a pop-up flash will fill in shadows and save you space. Take an extra-long telephoto and tripod only if you plan to shoot from shore. To photograph wildlife from the raft, ask the trip leader to move the boat to an eddy (or another calm part of the river), and use fast film with a telephoto lens in the 80 to 200mm range.
A polarizing filter will reduce glare from reflections, darken blue skies, and increase color saturation.
Outfitters usually provide metal ammo boxes to keep your equipment dry and safe. Discuss your needs when you make your reservation. On overnight trips, see if there's enough space for you to have two ammo boxes - one to store your extra equipment, the other to keep close at hand with film, lenses, batteries, and a lens-cleaning kit.