Because aquamation involves heating water under pressure, there is certainly an energy input in the process, but it is significantly lower than that of flame based cremation. The total amount of energy used is only 10% of the energy that is used for flame based cremation.
Since there is no combustion or smoke stack, there are no emissions of mercury, CO2, or particulate matters from the direct process (for carbon geeks, there are no scope 1 emissions in the process). Almost all the energy in the process is just the energy used to heat the water.