How much water do you want to heat, and how big a battery? Heating water takes a lot of energy and will require a very large battery. No circuit can get around this limitation except for a Peltier heat pump, which can make more heat than it takes off the battery because it uses a cold sink that pumps some heat out of the air too. The benefits are not earth-shattering.
Generally there is no "schematic" because it's just a heating element, a resistor. There are 12v coffee heaters out there which sound like what you want. Nowadays they're usually "hot plate" designs, there used to be immersion type but they're no longer common because of the cleanliness issue.