I was screwing around the other day with an extra 2.5G tank that I had laying around. I popped my camera into the tank and set the tank into the top of my 120G tank. Granted, you gotta be careful and make sure that you don't submerge the 2.5G tank too far. I got a couple neat pictures, but didn't spend a lot of time playing with it.
I've been tempted to make a little 4"x4"x8" (approx) acrylic box that will fit over my 100mm macro lens that I can just pop the bottom pane into the water.
Eric