Hi, Some How-To Arduino Power info here: http://arduino-info.wikispaces.com/ArduinoPower
Water and heating elements have a lot of "thermal inertia".. If you use a Solid State Relay and turn it on and off about 5 times a second, and vary how long it's on each time, you can control the heat input easily. Example: http://goo.gl/jbNwl which is optically isolated so you have no direct connection between your Arduino and your heater..
DISCLAIMER: Mentioned stuff from my own shop...