1. Coal. Set it on fire, use fire to boil water
2. Geothermal. Go down where the rocks are hot, use hot rocks to boil water.
3. Nuclear. Magic rocks get hot all by themselves. Use them to boil water.
4. Photovoltaics. Shape rocks into solar panels, use solar panels to power stove to boil water.
5. Concentrated solar. Use mirrors to reflect sunlight onto salt (a rock). Boil water with hot salt.
6. Put water in a glass tube. Use mercury (which comes from a rock) to draw a vacuum. Water boils at room temp under a vacuum.
7. Lob a space rock at the planet. Space rock vaporizes everything in a 100 mile radius, including water.

I’m sure we can think of more