Everyone has already covered most of the important sanitation / safety issues so I'll move on to other items:
You can have fairly wide temp swings doing this; the smaller the volume of water the faster the system is going to respond to the heat that you add. Play with it and if you really like it, consider putting the coil into a small insulated drink cooler and adding a small electric heat element with 120v power and control the whole thing via Chinese PID and you will have a pretty nice, electric controlled HERMS system. (Will hold heat and respond quick enough if you have a small volume of water.)
Adam