This is a floating dock that's easy to make and works beautifully.
Here is quick parts list of everything I used:

4 - 2x8" pressure treated lumber. 8 feet long.
7 - 2x4" pressure treated lumber. 8 feet long.
17 - 1x6" pressure treated lumber. 8 feet long.
4 - 4x4" pressure treated posts. 8 inches long.
4 - 55gallon plastic Barrels
100ft of Rope
16 - Screw in Eye Hooks
10 to 20 - L shape braces
Galvanized screws and Nails
Drill/Screw Driver
Hammer
Silicone Caulking

UPDATE
Here are the parts for the ramp/walkway I added (Step 7)

1- 55gallon plastic Barrel
2 - 2x8" pressure treated lumber. 12 feet long.
2 - 2x8" pressure treated lumber. 3 feet long.
2 - 2x4" pressure treated lumber. 12 feet long.
3 - 2x4" pressure treated lumber. 33 inches long.
4 - Screw in Eye Hooks
6 - L shaped corner braces
1" steel pipe...length depends on water depth.
Rope...screws...hammer.......

Step 1: Build the Frame

So once you have your parts head out to the body of water you want to place the dock in. Get as close as you can as your final product will be quite heavy and you don't want to have to move it too far.

Luckily I had a relatively flat area near the lake I was placing mine and I could keep things pretty level.

Lay your 2x8 boards out in a square and screw them together. Make sure and keep two sides of the square on the inside of the square effectively making an 8'x8'4" square. I used the pieces of 4x4" posts in the corners as a right angle. I didn't screw these in yet incase I needed to make adjustments.

At this point you can also ready your barrels. Make sure the plugs are tightened and then apply a layer of silicone caulking over the plug to ensure a good seal and prevent leaking. I did not fill the barrels with any sort of ballast but some people suggest doing so for stability. But my final product was quite stable so no worries.

Step 2: Support the Frame

Now that you have the basic shape we need to add supports.

Measure out the middle of the square and place a 2x4" support there.

The remaining bottom layer pieces are playing two roles. They are supporting the frame as well as holding the barrels against the dock and preventing them from pushing up against the decking. Place two of the boards as the picture shows and lay a barrel on top. Shift the boards around until you get the barrel sitting nicely in between the boards without touching the ground but also fitting nicely around the curve in the barrel. Mark that point, screw them in and do the same to the other side.

Now the top layer of supports that run perpendicular to the rest also have two jobs. The provide the cross support and keep the barrels from moving back and forth while in the water. So once again place your barrels on the bottom supports and measure where the barrels end. Place your top layer supports here and screw them in.

Step 3: Add the Barrels

After screwing in the 4x4" post pieces in the corner to solidify the structure you can make things a little more stable with L Braces places at each of the support intersections. This will make sure that everything stays where it is and really tightens up the frame.

Now, place eye hooks in the bottom layer of supports where your barrels lay. Two on each side the barrel. Lay your barrels in their slots and tie them up! I started on one eye hook with a knot and ran rope across the barrel, then diagonally, then across again and tied it off on the last eye hook. After you do all 4 barrels you are ready to flip.

Step 4: Flip It.

I did everything up to this part alone. Now I recruited some friends to come and help me move the beast. With only 3 of us total we easily go it up on it's side and slid it in to the lake. I'd say we might have been able to do it with just 2 of us but 3 worked nicely.

Flip it in to the water and tie it down to something. You don't want it floating away while you go inside to get the beer.

(It just rained for a week straight so the water level was quite high here, hence the dock basically floating in the grass.)

Step 5: Deck It.

Now that you have a friend or two...and your beer, this part is a breeze. Lay out the 1x4" boards and make sure everything fits nicely. Leave a little spacing between each board. Drink your beer and hammer in the boards along each support. Once you get a few in you can jump on top and finish from there. Feels nice and stable doesn't it!

Step 6: Float.

One thing that is nice about a floating dock is that you can untie it and float out in to the middle of a lake or pond for a nice swimming platform or just a nice place to hang out.

I'm still working on a ramp and a good way to keep the dock from drifting while actually on shore. I'll be sure to post that once it's complete.

Step 7: A Ramp Begins...

So now with the floating platform/dock I needed a way to easily get on to it without stepping in the water first, so here is the walkway/ramp.

I'll go through it quick as it's basically the same thing as the platform.

Working with this was different in that the 12 foot boards are much harder to get level where I was working as the slope of the ground really came in to play with the extra 4 feet length. So I used a lot of extra boards and stuff lying around to try and keep everything flat.

Basically though it's the same idea. Frame the entire ramp with the 2x8" boards. Screw them together and level. Add the 2x4" boards down the middle as support for the barrel as well as the walkway area when you flip it over. Throw in the 33" pieces of 2x4" board for support every 3 feet. Add the Eye Hooks for the barrel, rope it down and you are ready to flip it.

I did this in about and hour and a half. Most of that time was spent making sure it was level...and it still wasn't perfect.

Step 8: Flip It and Float It.

Once again, bribe a friend with beer (or in my case my friend actually brought the beer) and flip the ramp over and walk it in to the water.

Now, this is where this could easily not work for some people depending on their shoreline. Mine has a nice little hill the ramp can sit on while on land. So I set that end down and the other side floated nicely out by the platform. I had to add some dirt underneath the land side just to get everything straight but it wasn't too bad.

Now just put the decking on just like before...thanks to those 2x4"s running long ways you have plenty of support for the deck boards.

I actually ran out of boards for decking because I wasn't really thinking...remember, after wood gets milled and sent to the store it is no longer it's original dimensions...I calculated for 1x6" boards, when it reality they are more like 3/4x5 1/2"....so I came up short.

Step 9: Attach and Secure...

Now it's all finished but the platform is floating free and the ramp is somewhat unstable. So I placed poles (not pictured) on the corners of the platform and sunk them as far as I could in to the lake bed. I also placed a few on the ramp to keep it from getting pushed to the side. Just don't make anything too tight, everything needs to be able to move by wind, people walking, water level changes. I've already found the poles make great places to lauch bottle rockets from as well as place tiki torches! (I'll throw a pic of that in soon)

I also attached the platform to the ramp with rope, carabiners and eye hooks. Check out the pic on how I did it. Now it can still be easily detached to float out if need be.

It's not as stable over all as I hoped...if you walk to the far corner of the ramp it dips pretty low in the water and actually rolled the ramp before I added the poles. I guess I could just add 4x4" pillars and take out the barrel........maybe once the water level goes back down to winter levels.