I do not know your location and what waves and tides actions you have.
If you are inland and you are looking to build floating docks on the cheap
to last for a year or so with ease of handling your approach could serve
you on a short term basis.

About using used plastic barrels, check where they coming from and what were
they containing.
I had to give up the idea of using plastic barrels because they had been
used for the transport of toxic chemical.
With today's environnemental bylaws it would be safer to check with the
proper authorities.
The following suggestions are subject to you area and waterway regulations.
In some location the design will have to be much stronger to meet the marine
environnement of were you are building the floats.
As for the construction of the 3' X 16' floating docks 2" X 8" rough sawn
juniper is on the light side.
What I used was 3" thick by 10" wide. This size is not easy to get by, but
if you have access to a local (or portable) lumber mill it is no problem.
As for the decking 1/4" X 3" is light I used 2" thick by 6" wide.
You said you doing this on the cheap. I suggest that you use steel corner
plate bolted through the cross member and the side stringer. Nailing could
do for a short term but I do not recommend it.

ups.com...
I am going to build some floating docks but before I re-invent the
wheel I would like to get some opinions. They will be 16' long by 3'
wide and at either end I would put a plastic barrel under it, inside
the frame. The frame would be 2X8 rough sawn Juniper and the decking
would be 1 1/4" X 3 . First of all would this be stable or want to
roll, should I put a 3rd barrel in the center? If the barrels were in
a framework but free moving could they act as rollers to help when
pulling the docks out of the water. The beach is very gradual. I am
doing this on the cheap so I am not buying any floating dock hardware,
brackets, plastic floats etc...Any other ideas would be appreciated.
Thanks