I have been contemplating using one in my tank as well. My thoughts are that the grease in the flex tubing could possibly get into the water and cause some harm to the tank so I have decided against doing it. Just my .02.
IMO no. I have one and it looks like there is no way to keep water out of it. They water would enter where the bit attaches and you can't wrap or seal that. You might get a couple uses out of it but would rust and bind up in the end. Also i am not sure what oils are inside the flex shaft. I wouldn't risk it.