Aqueous fluids are heated or boiled in a tank or vessel by causing cavitation in the fluid within the tank or vessel. A rotor having cavities on its cylindrical surface is rotated within a closely dimensioned housing submerged in the fluid, deliberately causing cavitation which heats the aqueous fluid without the use of flame or heat exchange surface. An electric motor which powers the rotor may itself be submerged in the tank or boiler vessel. The rotor includes radial channels for imparting centrifugal impetus to the fluid as it flows toward the cavitation zone.