The depth of the water does depend on the size of the turlte. Make sure that your turtle can reach its head out of the water while still being able to stay sitting on the bottom of the tank. That is the proper height. As it grows, you can start adding water height.
I have a common snapper and I do use gravel and provide him with a basking area. He really enjoys digging and burying himself in the gravel. I think it helps him feel protected and gives him something to do. His basking area is made consists of three rocks. 2 wide ones and one thin, long rock. I put the two wide at the bottom of the tank and the skinny one spanning across the top of them, creating a bridge. I would estimate my snapper spends about 50% of the day time basking on the rock. And most of the remaining time sitting underneath the bridge. The rock setup really works well.
I have heard of snappers basking constantly and some not at all. It is something that you will have to experiment with. Definitely have the majority of the tank comprised of water and just one bask area.
A heater is not a must. As long as the water stay around room temperature it is fine. Snappers are very hardy turtles and cooler waters do not have an effect on them.
Hope this helps.
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Mike
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2 Painted
1 Musk
1 Common Snapper
1 Cooter
1 Map
1 RES
1 Corn Snake
1.1 Water Dragons
2 African Frogs
1 Toad
1.0 Degu
1.2 Chihuahuas
0.1 Cat