This page has results from testing different UV tubes. It shows the 10.0, at 4 inches, outputs about 110uW/cm^2 of UVB. After 105 hours, this drops to about 100.
at 20 inches it's output is is less than 20uW/cm^2. (not much moer than 5.0 bulb). from what i've read, a minimum is about 60uW for most herps, but having more usually has nothing but positive effects on the animals (unless you go crazy, into the 350+ uW/cm^2 range, but this is only possible with MV bulbs).
with a tube UVB bulb, closer is usually better. this holds true for your water dragon. about 6"-8" from this bulb is probably your optimum distance