You're thinking too much OP, from my understanding you have the water heater and socket for water heater already in place, just need to connect the water heater to the socket, which is like 1-2 feet away?

The 70/0.193mm cable should work, if you're paranoid then go buy cable with even bigger diameter. If you're still paranoid, after your long shower go and feel the wire whether it is overheating or not..