I always tie a soda jug to the top of a cedar tree as well as a rock or concrete block to the bottom; that insures that the tree won't fall over on its side. Another, longer lasting method of creating structure is to fasten several tires together. And you can't just pile them, or even stack them like you would when storing them on shore. Take the first tire, stand it upright, and fill the bottom part with concrete or gravel; concrete is best because you don't have to worry about it falling out while you're transporting the tires. Drill several holes in the top part of the tire to let trapped air escape. Take the second tire (in an upright position) and stand it on top of the first tire at a 90 degree angle to the first tire. Drill a hole through both tires and bolt them together with a nut, bolt, and washers. Drill holes in the top part of the tire to let trapped air escape. Continue the process for as many tires as you want, adding each tire at a 90 degree angle to the tire below. Do not drill air holes in the very topmost tire; also, shove a 2 liter soda bottle into the top of the topmost tire to help hold the structure in an upright position. You can also use old shipping pallets to make fish habitat. Just fasten several together to make boxy shapes and add weight to sink them. Here is the method used by one group of people using pallets: The first step is to accumulate wooden pallets. Pallets are donated by various companies in Wisconsin and trucked to the flowage at our expense. Cinderblock, clips and strapping must be purchased to build cribs. Cribs are constructed by layering pallets separated by cinderblocks on each comer until it is 3 or 4 pallets deep. Then the "sandwich" is strapped together. At this point the cribs are loaded by tractor-forklifts onto special pontoon boats that have been totally stripped down to just bare decks. Workers then turn the pallet/cribs on edge to stuff in brush. It takes a lot of brush to fill each crib. Accumulating brush and stuffing the cribs is actually one of the hardest parts of the job. Finally , when the pontoon is totally loaded, the captain and workers shove off and the cribs are dropped off into new crib locations.