I would not recommend making the water too deep for now, especially until his rear legs are back to normal. You don't want to stress him out more by making him overexert himself trying to swim too much. In general babies are usually good swimmers right off the bat, but they still need an area that is shallow for them to rest. This can be some plastic plants that come up to about an inch or two below the water's surface, a rock, or a sloping ramp to the basking platform.
The 85 degree temperature seems much too warm. Check the water temperature of the stream you have nearby. I doubt it's that warm at all. 80 degrees is a recommended water temperature for a sick turtle since it will help boost their immune system, but if kept at that temperature for long term, it will encourage too rapid a growth rate which may not be healthy. The basking spot should definitely be about 10 degrees warmer than the water to encourage the turtle to come out and bask. The basking temp should be more like 88 to 90 degrees.
There are a lot of different sites out there with varying degrees of difference in their recommended guidelines for proper RES care. Some are better than others, but many just offer a different method that has worked for them. The key is to find what works best for you and your turtles and make adjustments as necessary if your turtles don't respond positively to their setup.
Personally, I've found the information on this site to be the most accurate and in line with basic common sense. I also think that we should try to replicate their natural environment as much as possible. Since the natural habitat of the RES covers a fairly large area of the US, we have some flexibility here.

But the babies are the ones that need the closest attention to make sure everything is just right.