12v nichrome wire water heater. Is it safe?
No, it doesn't seem to be safe. You have full potential gradient accross the wire immersed into water. Therefore, you have to consider electrolysis reaction where some interesting compounds may emerge depending on impurity of water. Even if you make it AC, I don't think that all of the reactions will be reversible with each alternation because of possible convection currents of liquid. So, isolation is required.