I second the idea of adding a separate shallow glass container as a pond, then raising the substrate/bark in the larger tank until it is level with the edge of the "pond". The big advantage in this is that you can easily remove the "pond" for cleaning and water changes when necessary. Look for a Pyrex or Corning Ware cake/brownie pan if you want a rectangular shape, or a casserole dish for oval shapes. If you want a round "pond" there are a variety of sizes of round Pyrex containers for baking and/or storing leftovers. If you have a glass shop near by, they might be able to cut down the glass aquarium so that it is only a few inches deep; provided it will fit inside the larger cage (I can't quite see from the picture).
The other option is to fill a used aquarium with a few inches of water, and then use rocks and sand to build an island that the turtle can crawl onto and bask himself.
Sliders are aquatic turtles, spending most of their time in the water when feeding. The come out of the water primarily to sun themselves and regulate body temperature, as well as to lay their eggs. I had a tiny painted turtle as a pet for many years when I was a kid (he started out about the size of a 50 cent piece), and he spent most of his/her time in the water. Turtles are prone to disease if they aren't healthy; so you'll want to read up on proper care and feeding.