For what it's worth, here is a crude sketch out of my files of a platform I constructed several years ago out of 6 barrels. If you can see by the drawing, the barrels are held in place on the 4 sides by 2x6 treated boards that are positioned and cut to the length so as to remain approx 6" below the centerline of the barrels. Those bottom support 2x6's were joined to the top with treated 2x4's. The deck (treated 1/2" plywood) rested on the tops of the barrels as well as the floor joists (also 2x4's) which are attached to the end joists that form the perimeter around the tops of the barrels. The end joists are positioned and cut to assure they line up to make sure the plywood deck lays exactly on the joists as well as the tops of the barrels. the deck is then screwed down on the joists. I built the sections on land, took the barrels, two sections, the joining 2x4's, decking to the water and put everything together there. I floated the bottom section, put the barrels into the bottom, put on the top section, had two buddies lift each side, one at a time and screwed the supports to the top section. Put on the plywood decking and screwed down the decking to the joists. If you have some strong buddies or are able to use a lauching trailer, you could build the whole thing on land or do as I did but have the decking already on the top section.
The bottom section, being below the centerline of the barrels keep the barrels from skewing and/or floating away and is the way of binding them together without bands, etc.
Don't know if this was what you were inquiring about or whether you already have your plans designed, but this worked. Many other ways to do this. The other things, such as achoring, making it steady with floating dock type driven poles with rings or pipes for fluctuating water levels, etc are all pretty easy to install.
Good luck. Let's see some pictures when you get it built or in progress.
(Tried to post sketch but can't seem to get it to attach)
Last edited by tenfingergrip
on Tue Jul 30, 2013 8:50 am, edited 1 time in total.