>>512682322
Those thick plastic drums will take a nuclear blast and last decades, you just have to seal them well. Problem is the structure. Traditionally you'd cobble some bullshit together out of skids, lash it all together with nylon rope and call it a day. Smarter way would be a grid frame made out of something decently flexible but non-degradable, with removable deck pieces to reattach lost or damaged barrels from above. Have the foresight to attach a few rollers to the bottom and you can just drag it on land at any boat launch.
Rather than make larger platforms, go modular and lash smaller ones together with some give, then have bolt holes to affix rigid beams for when the water is calm enough to motor the whole lot around together.