I had the Duck out for an autumnal paddle yesterday and found that I was able to access the hatch even out on the water. The trick is to slide forward into the cockpit, roll over onto one's stomach, then slide back toward the rear. You end up with your chest on the seat back, head and shoulders over the rear coaming. In that position the rear hatch is easily accessible. It was nice not having to beach to get at the stuff in the hatch.
Note - only try this on still waters. While the WD12 is incredibly stable, you don't want a wave breaking over your rear with the hatch open.