Another idea I've read about and am about to build is a tire structure. Piles of tires can work too, but this sounded better. It requires 12 old tires with some consideration for the size of the tire as you will see.
Take four tires and screw the sidewalls together until you have made a cylinder of sorts with them. Do the same to the other 8 tires so you have 3 cylinders of 4 tires each. Lay 2 of these cylinders side by side and screw the treads together. Take the third cylinder and put it on top of the other two in the middle where it should sit on its' own, and screw through its' tread into the tread of each of the other two cylinders so that you have a large, stable triangle or pyramid of sorts. That's it.
Well, almost. You still have to get it where you want it and sink it since it will float if left this way. One way would be to put a little concrete or rocks in the bottom of the two lower cylinders. I'm also toying with a silly idea I dreamed up using a portable drill and strategically drilling holes once I have it where I want it, to get it to sink there. But I'm not sure it'll work so I won't go into that.
Fish would be able to swim through all three tubes created by these tire cylinders and hang out in the shade inside and out. I gotta think a big one or two will claim the inside of these cylinders as their castle!
