The correct answer is - The water was heated by magma or hot rocks.
The hot water that is erupting from the vents comes from the geologic activity inside the Earth. The water that is deeper into the ground, where there's geologic activities around the area, gets heated by the magma or hot rocks, was results in increasing the pressure, thus the water is shoot out in the air, looking like the vents are erupting.
Most of the vents like this can be found in areas that are close to a volcano, or are connected with a volcano through underground channels.