You can make the platforms for your dock from several layers of common home insulation foam. Look for something 2" thick or thicker. Cut each 4x8 panel in half lengthwise and use a construction adhesive to build up a block which is 2 feet wide and 12 inches thick.

Drill a hole through the blocks about a foot from each end and force in a pipe running through the width of the block. A wide "T" fitting on the end of one of these allows it to ride up and down on the pipes you drove into the riverbed. A smaller "tee" fitting on the other pipes allows you to run your stainless steel cable through to link the block together, (but allow them to flex)

Cover each 8 foot block of foam with 2 layers of 6 ounce fiberglass and epoxy resin (which is then painted) for greatest durability, or use an elastomeric roofing material and open mesh fiberglass roof patching fabric to paint over the blocks. You either end up with a rubber coating, or an epoxy one.

You can use construction adhesive to attach two 8-foot-long 2x12 planks to the top of your foam blocks. If you want, you can use long "U" bolts going all the way through the foam. This serves as a mechanical tie, adding to the construction adhesive holding the foam slabs together.

to be continued in part 4--if that gets through the spam filter