Used 55 Gallon Tight Head drums are great flotation devices for building floating docks. They have buoyancy rated up to 440 pounds
For sale here are 55 gallon Plastic Barrels that previously held liquid food condiments such as: Soy sauce, hot sauce, vinegar, margarita mix, etc.
Food Grade 55 gallon "Tight Head" Barrels. These barrels do not have a removable lid but come with two - 2" white caps with rubber gaskets that create an air tight seal. They are great for many of functions around the House, Ranch, Farm, or Garden. They can be used to store, haul or distribute water to hydrate livestock. Also commonly made into: composters, rain barrels, troughs, vertical gardens, floating docks, and several other uses.
Again, retail pricing is $25 for an unwashed barrel