I had the same types of questions and built what i think is the best and cheapest way to go. I bought an old boat trailor 20 ft long. I ran 2x8x16 lumber down the sides on the frame of the trailor and bolted it on leaving the 2x8x16 sticking above the top edge of the traior 4 inches. I started at the front of the hitch and made a 3 ft walk way using green lumber screwed in with deck screws and went up 8 feet. The deck then was wider at 12x10 ft with a two foot over hang. I treated the lumber with heavy linseed oil and really soaked it in and then put a nice stain on the entire deck. I then got some plastic 55 gallon drums and attached them under it using metal straps and sucked them up tight to the wood frame. I stuck her in the water and it floated just as had predicted, very unstable. By the way i built it for the same purpose that you mentioned. Fly fishing off of it and the kids can sun and play on it. The plan went hay wire when two of us 180lb guys got on it, it tried very hard to sink. I had a back up plan however and it turned out wonderful. I got in the water and lifted the dock up and put a cinder block under the tires and brought it up to the level i wanted. So you have the support of the barrels as well as the support of the blocks. You can easily get 6 adults on it and have a good ole time. As far as the front of the trailor popping up with all the weight on the deck, i then screwed a trailor house hold down screw into the ground about 3 ft using the chains on the hitch as an attachment and it will never go anywhere. Total cost was $200.00 If i choose to i can unscrew the hold down and hook onto it with my pickup and pull it out in a few years and re treat the wood. I doubt if i will ever need to. It was a fun project and if a guy could make them in large numbers i don't think i could make them fast enough for the demand. People have asked me to make them one and i don't really have the desire but told them how easy it was to do. You can go to any boat dealer and they have old trailors sitting arround and you can get one for nothing. I had this one hand so i didn't include that in the cost. I also put a stainless steel grab bar at the end of the dock so the kids can grad hold and get on. Sorry for being long winded but this in my opinion is the only way to go. If you want a picture of the dock in the water let me know and i will email you a picture.

Jeff