hmmm, almost a month late in replying. If you are still checking, then here you go.

If you're Red-spotted Newt (now called the Eastern Newt) is in it's Eft stage (juvenile) then the water dish should be shallow as they are a terrestrial species at this stage and if they get trapped in water then they can absorb it through their skin and bloat to death.

If you have the adult which is aquatic then depth isn't a huge issue as they will swim up to the top for air whenever they need a gulp. Any average aquarium won't get too deep for them. The captive one's I've worked with are in about 30 gallons of water roughly a foot deep (real guestimation). If you have a ten-gallon tank then fill it up about 4 or 5 inches away from the top. 20-gallons as well. If you want to be real generous you should have somthing sticking out of the water that they could climb up out of the water onto if they need to. While I don't think it's a requirement, I've seen adults come out of the water from time to time and haven't yet figured out why. In the wild there are plenty of reasons why they would do this but none of them apply to captivity so it's probably just instinct and you can either make room for it or ignore it. If anybody knows otherwise please jump in...