There is heaps of stuff on the net, and several books on keeping Water Dragons in captivity. many of them suggest that as they age, they change from insectivorous to mainly vegetarian, but my experience suggests that the adults still prefer insects - I have a huge supply of woodies in my compost heap, so that's my main food source for them. My good friend Peter Harlow who probably knows more about them than anyone, basically fed his on canned Whiskas Jellymeat for many years, and his large male died when it was about 28 years old. Fantastic lizards, greatly underrated, you'll get a lot of pleasure from them, but if you are able to, once they are big enough, keeping them outside is far better for them and probably a lot less work for you. Mine are outside, and while it is fairly cool here at the moment, they are still active during the day, but have stopped feeding about a fortnight ago.
Books - "Keeping and Breeding Australian Lizards" edited by Mike Swan, and "Keeping Australian Water Dragons" by Jason Goulding and Darren Green, are both good sources of info.
Apart from the larges size attained, I don't know why these spectacular lizards aren't more popular. I guess it's because they are common and cheap - if they came from South America or elsewhere people would be queuing up to buy them.
There's also an EWD keeper's site on Fb.
Jamie