Larkin you are probably correct about those other turtles, I'm only really have experience with Western Painted turtles, and a little bit with red eared sliders, I was just repeating complaints that I've heard from people in some turtle forums, and yes, I believe they were referring to soft shelled turtles, which are commonly kept in aquariums where they are highly visible and observable, but in a pond setting virtually invisible because they hide under water all the time.
Mike thanks for posting the picture of your pond, I like your waterfall feature, could you post a close up picture of it?
It looks like you have a tall brick wall on at least one side, and maybe a fence? on the right. It is unnecessary, nor desirable, to build a barrier directly around the pond. As long as you have your yard secured so the turtle can't escape and wander away out onto the streets or off to the neighbors, it is ok if they wander around in the yard a bit. Any other barrier you need to build only has to be about a foot high as long as they can't climb over or dig under it. The biggest danger of having them wander around in your back yard is that someone might inadvertently step on them or you might run over them with the lawn mower. Other than that they would probably appreciate the freedom and space.
These turtles might not be too bad, all though I don't know if they would suited to your winters. Looks like you have researching to do.