Look at dock floats, they are way more stable.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=431675&page=5I still have mine gangways setting on concrete blocks and tied to ground anchors. I can use one to set out trees with a small trolling motor. Each 12x12 has 5500# capacity and very stable, it's like a small barge. I can also configure it in many different shapes but so far I haven't.
In a 1/4 acre, a 4x12 gangway and maybe a 8x8 would work great.
For anchors in water I used old tires filled with concrete with eye bolt in the center to attach chain. They were 160# each, but easy to roll to where I wanted to place them. Left anchor was attached to the right side of the dock, and right anchor to the left side. Crossed underneath, so don't have to worry about someone jumping on the chain.
