Yes you can drink tonic water and it can be healthy in some ways as long as you do not drink too much tonic water.
When drinking water you also need to drink plain water to keep your kidneys and other organs healthy so that you can prevent kidney disease and other health issues.
Drinking plenty of water also helps prevent gout and you'll also feel fuller so you won't want to eat as much which means you'll lose weight as well.
I've drank some tonic water before but I didn't like the taste but tonic water can be consumed and it will not harm you.
As long as you don't drink too much tonic water as if you do you might get an upset stomach or bloating in your stomach.