Measure the barrels. Typical 55-gallon plastic barrels are just under 34 inches tall, and just under 24 inched in diameter. Use these dimensions for this platform project. Adjust your measurements as needed.
Examine the barrels. If the barrels are not sealed, seal the threaded fittings on the barrel with a water-tight lid, then apply a heavy bead of silicone around the lid.
Assemble a frame for the platform from the 2-by-12 pressure treated dimension lumber, 80-inches-square. Stand the 2-by-12s on end, and build a simple square box by nailing the lumber together at the ends. Use twisted, “Ardox” nails, or ring shank nails to hold the lumber together.
Cut two pieces of 2-by-6 dimension lumber to 77 inches long, and nail them into a frame, creating three long rectangular spaces inside the frame, each 25 1/2 inches wide. Cut three more pieces of 2-by-6 dimension lumber, and nail these inside of each of the long spaces, thus bisecting them evenly. Fasten all the 2-by-6 lumber so the pieces are flush with what becomes the top of the platform. You now have a rectangular frame with six open areas. Each open area is just larger than a 55-gallon drum.
Cut 8 pcs of 2-by-4 treated dimension lumber, each 8 inches long. Nail these short blocks onto the frame, along the outside of the 2-by-12 frame. Each open area holds one 55-gallon drum. Nail the braces so they are attached to the frame where the side of the barrel is exposed. These braces keep the barrels from sliding out from under the platform.
Flip the platform over. The 2-by-4 blocks now point toward the ground, like short table legs. The 2-by-6 dividers are flush with the top edge of the frame, which faces up. Cover the top of the platform with the 3/4-inch treated plywood. Nail the plywood down to the dimension lumber, lining the seams between the pieces on top of one of the 2-by-6 dividers.
Cover the top of the platform with indoor/outdoor carpeting. Wrap the edges of the carpeting down over the edges of the platform, and staple the carpet in place.
Turn the platform back over, and place one barrel in each of the open areas. Using the steel wire, wrap the wire around the bottom side of the barrel to hold it in place. Put a couple small nails, or staples, into the 2-by-6s to hold the wire. This wire holds the barrel in place while you transport the platform to the water. When the platform is in the water, the weight of the wood keeps the barrels in place.