I've done a couple 30 day trips in the last few years and used pelicans both times. Each trip had a fair bit of white water, but infrequent and relatively short portages. The one long portage (about 2.5km) I simply carried the pelican in my hand. Wasn't a problem as you're typically carrying dry bags or paddles in your hands anyway. The pelicans did give easy access in an open canoe the second year, with a spray deck though it's more difficult. No one on the first trip had a great solution for this. Perhaps a topload zoom case with a couple small primes in a drybag would be the best option for when paddling. Then the rest of the gear remains stowed until you can stop and get things out.
I've got a ~5" deep storm case (IM2400) which I used the first year (snake river, Yukon) with a D200, 20mm f2.8, 50f1.8, and 70-200, along with a folding solar panel, two flashes and triggers, chargers for camera and batteries, and a hyperdrive. This one was fully submerged for about 20min in a swamped canoe after not quite making it past a class III rapid. Only a couple small drops managed to get in through one of the corners from this.
The second year (Nahanni river) I also brought a pelican 1560. This 1560 held a D3, a D300 on a 200-400 f4, 20mm, 50mm, tc14, two flashes. The Storm case I think was holding mostly chargers and cables, two solar panels, filters, hyperdrive. For the D3 I had to have a small sealed lead acid battery pack with an inverter which was in a small lowpro case (6"x6"x9") which I would stuff into one of my drybags. That battery pack managed to charge two D3's (one was my buddies [I had two batteries, he had 4 or 5), a canon 40D, my hyperdrive, and his storage harddrive for a month.
If you want something accessible all the time, probably a Canon G1x in Canon's waterproof housing would be handy for while paddling. Then pull the bigger slr stuff out when you stop.
attached is an image of the open canoe with the 1560 at my knees and the storm case upright behind my seat. For this trip, as I was in a solo boat, the other 18' canoe carried most of my food. This set up, at your knees, might work if in the back of a tandem boat.
This image was actually taken with a D3 mounted on a magic arm to the back of the boat (overhanging the side), with the intervalometer set to every 5 minutes or so.
