Wave splashed or thrashed shorelines are always clean which I attribute to the frequent oxygenation and agitation of those shoreline bottom zones.
Instead of a boat anchor that is drug or pulled by your tractor consider using a bigger piece of homemade tiller such as bed springs, a chain link pipe frame gate, a small field drag, a delta harrow, small section of a spring tooth drag, a spring tooth harrow, or a homemade, square-rectangular weighted stove bolt drag made of 2"PVC about 4ftx4ft with 4" to 8" bolts spaced 2"-4" apart on the back side opposite the pull line or drawbar.
http://ourtinyfarmnc.blogspot.com/2011/12/build-your-own-drag-chain-harrow-manure.html 
