Sorry for being a bummer here but: VERY BAD IDEA. Chinese water dragons are very cool and personable lizards but if there is some things they are weak is: water borne patogens, too much moist and cooler temperatures, all are in the pacage with your turtle. First and more importantly, your turtle´s water, unless it was either WASHED EVERY SINGLE DAY period and even then the risk would still be present, its totally unsuitable for a water dragon. Your dragon would still take its normal baths with your turtle and could get really sick in the process. All bacteria that are normal in turtles could be letal if they get their way into a water dragon sistem, stuff like Amoeba and Sallmonela, DEADLY to water dragons and basilisks NOT FUN. Saw it first hand. That is why for example in a huge habitat you can put lizards and tortoises together but not turtles. The people that do such combo are playing with their animals health. Water dragons like all lizards need allmost human grade water in order to be healty.
When I had water dragons many years ago one of the things I learned and that is vital is that you have to change the water every day because they poop in the water basin and if you dont clean it the water becomes fowl very quicly. Now mix that with a messy turtle...not gonna work.
Next water dragons may like to swim but...they can drown and deeper water (in the likes of your turtle) are not for them,. Also in a terrarium kind of habitat your turtle water basin is shure to cause condensation in all materials present and specially during the nigth, completly fine by the turtle but horrible for the water dragon. Water dragons not only need to sleep dry but they also need a substancial part of their habitat to be dry, or else they develop blister skin desiase really quick.
As you can see its unfornatly a bad idea to comb both animals and that not considering that this turtles are just like any other prone to bite your lizard. If you do it its very likely you come to regret trying it in the first place, with bad stuff going to the water dragon. These are owesame lizards but if you want to try them you must also understand they are on the fragile side. On a encoraging note I will tell you this however: as it seems australian water dragons are MUCH hardier in therms of water quality. Bert told in its web site that this were the hardiest lizards conserning parasites. I know of people sucessfully keeping them with turtles in both HUGE outdoor exibits and tropical green houses. In this kind of space with both species having more then enough space and with a pond well filtered its possible to acheive this interesting combo. This is what you should aim for. In the mean time defenetly keep both species separate in diferent terrariums.
