Eastern OK is very nice as it still has the hills. Western OK is a different animal. It does beat Flatistan, but don't quite knock this State as we do have some very nice riding area's too.
In as far as the water crossings go, we mostly took it very easy and usually got of the bikes and checked it out before rolling through. We skipped parts of TN on our trip as, I hate to say it, got bored with riding the pavement and the diversions just to get to a mile of dirt. Consequently we didn't do any water crossing that I remember there...
TN is really nice though and I still would ride there but only if time is not an issue.