I don't know anything about water dragons.
Painted turtles are sometimes found naturally in brackish waters. They can tolerate it but I dunno to what extent.
Whether brackish or fresh, I would hesitate to keep a painted turtle with fish for a few reasons:
They will often attack fish, especially while the fish are sleeping, therefore not fully aware, whether they decide to eat them or not. They find the movement interesting and will bite them.
They produce A LOT of waste. The nitrates in your aquarium will rise quickly, especially in a smaller system (125g or less). Most fish don't tolerate nitrates to the extent that turtles will tolerate, so you end up having to do a ton of water changes to prevent the fish from becoming ill or dying. If your filtration isn't sufficient (a standard fish tank filter generally isn't sufficient) you will also have ammonia and/or nitrites present. If you have a large filter with large amounts of biomedia this is not a problem, but again, nitrates are.
And as a sidenote, brackish species of fish are often more sensitive to nitrates, so you may have more of a problem housing something like a turtle with brackish species.
That said, I've kept turtles in a 125g freshwater aquarium with fish, but constant water changes, filter cleaning, and water monitoring were a must. And I was aware that predation was a possibility, therefore the fish were mainly considered a food source.