I have a 12 x 12 dock which floats on 14 barrels, each 55 gallons. The barrels are sandwiched between 2 x 6's and float freely. I do think that the round shape of the barrels may reduce their effectiveness as compared to a billet. I started out with 8 barrels, two on each corner and found the dock tippy. I kept adding more barrels until I now have 14. The dock is very stable now.
As mentioned in previous post if the barrels spring a leak they sink. Replacing them is somewhat tricky. In the summer I swim under the dock and haul the remaining barrels from the outside to the inside to fill the space from the one that leaks. Then I take a new barrel to the edge where there is an open space and simply push down on it until it rolls under. The best way to do this (if you are 200 + lbs that is!) is to place the barrel adjacent to the dock and slide into the water off the dock rolling the barrel under as you go (kind of like a big lazy seal).
I have left this dock in the water for three winters and I have never had a barrel break from the ice. I have had on leaky barrel which was not placed in properly and banged on a cross piece until it cracked. I fixed it easily with plumbers goop.
I also have a swim raft which I got for free. It floats on $250 square plastic flotations. I left it in the water this winter and lost two of the floats, I think their square shape makes them more susceptible to ice damage. I would recommend the barrels, design the dock so you can leave them in over the winter and easily replace the barrels if any develop a leak. They are cheap and usually can be easily repaired.
Bill, I made two docks with barrels back in the late 1960s or early 70s and framed the dock so that each barrel was contained between two pieces of 2 X 6. But in a way that let the barrel drop out... more