Basking areas need to be provided in every tank and they should be easily accessible for the turtles.  This platforms must allow them to get completely dried out, including their plastron.  UVA and UVB rays are important for proper bone growth and maintenance.  Without UVB rays their body can't process vitamin D which can lead to bone disease.  Proper basking areas that allow them to dry out their carapace and plastron helps prevent shell rot and other shell problems.   Basking is also used to raise their body temperature.  Good basking will aid in shedding off old loose scutes.

In my hatching tanks I use floating platforms for basking area.  These platforms can be purchased at most pet stores.  

I do not like to use rocks for basking platforms for three reasons. If the pile of rocks shifts it can pin your turtle.  Adults tend to rush into the water whenever someone approaches they can hit the rocks causing them to chip their shells.  Lastly a more practical reason rocks take up more room in the aquarium than a floating platform.

In my juvenile and adult tanks I use wooden ramps for basking platforms.

Just like most things real sun is the best.  I suggest taking your diamondback terrapins outside once a week for an 30 minutes to an hour of real sunlight.  Make sure it is not too warm, as with hatchlings even adults can get cooked if the sun or basking area if it is too warm.