Water dragons are active, sometimes nervous little lizards. I wouldn't recommend keeping them with anything other than another water dragon, and then only a pair. Also, the waste produced by the turtle may contain certain bacteria that is beneficial to the turtle yet harmful to the water dragon or vice versa. Also, nearly any aquatic turtle has the ability to cause severe damage to a lizards body. Now add in different feeding requirements and communication barriers and you've got a recipe for disaster. For instance, if the water dragon is annoyed with the turtle in his area he has no way of saying, hey this is my territory. The turtle will have no clue what all that puffing and posturing is about and will ignore it completely resulting in physical confrontation. Also, to answer your other question yes that would be a good environment for a solo water dragon.