aqua - you got a baby!! i have 2 baby 3-toeds.
you don't want to have the water so deep that if they get flipped over in the water they will drown. i've had a couple scares where i came home and one was upside down in the water dish. For some reason he was having trouble flipping himself over, but luckily it wasn't so deep that he couldn't breath.
Here is a pic of my baby setup:
You can see the water is not very deep. I do give them a good soaking in some deeper warm water about once a week. I put them in the tub with one end pretty deep, and one end where they can walk. Gives them a chance to swim a little, which 3 toeds like to do.
For substrate I use a mixture of topsoil, natural reptile forest bedding (from pet store) and peat moss (from pet store). With all these tubs I've got it's about $60 every time I replace it, eek. So, right now my adult Ornate enclosure does look different because there is only topsoil in it. The adults are being dewormed right now, so I have to replace the substrate one more time. When I replace it again I'll go back to the mixture. I also suggest having the live plants. The turtles love them.
Also, I've read many sites that say keeping a box turtle in an aquarium or terrarium causes them a lot of stress. It's better for them to be kept in something with opaque sides so they can't see out.
Starchick and Steve - Yep, they're just zip ties. I drilled 2 holes on each side and ran the zip tie through. Also, under that astroturf are 2 more zip ties run the same way. Here are some more pics: