As much as you'd like to float your way across, you're almost certainly going to fall off and get wet. Or drown. They're not unrelated.
First, lay one of the planks diagonally, so that each end is touching each of the perpendicular sides of the moat. From there, rest the other plank on top. The first plank, acting as a foothold, allows the second plank to reach to the other side, leaving some room for overlap where the planks rest on each other and the bank.