Several years ago I built a dock much like you describe. 10' x 10'. Joists are 2x10s decked with 2x6s. We put twelve 55-gallon drums under it. Here is a photo right after we first floated it.
The platform just sits over the barrels that float between the joists. The are not held in place with anything but buoyancy and gravity.
The barrels stayed in place even when the pond has flooded, like during Hurricane Sandy.
This is the third or fourth winter for this dock. The ice has not affected it. It is held in place with two 3' deep by 8" diameter concrete filled holes on shore. The walkway is held in place, at both ends, with heavy duty fence gate pin ("hinge") hardware.
