Hey Bucks I have a floating dock just as you described. I can offer some advice. First off orient the barrels with a screw cap toward the top. Once the dock is floating you can unscrew the caps and add water to each barrel. What this does is stabilize the dock, otherwise when you walk to the edge, the dock will tip at a steep angle. About a third of barrel should be water. You can always pump some out or add more.
Attach the "gang plank" with two eye screws on the dock and two on the walk way with a piece of galvanized pipe through them. When the dock moves up and down this acts as a hinge. This allows for changing water levels also.
Cut a hole thought the deck in the two back corners and pound a pipe into the pond bottom to keep the dock where you want it. This way the dock level can change but not move side to side.
Lastly don't attach the walkway to the shore. This way you can remove the walkway by taking out the pipe that connects it to the dock. Your dock is now a raft and you can move it around the pond. I pull out the two anchor poles and "pole" my way around the pond like Huck Finn.
