You would do it with PWM (pulse width modulation) of supply to pump motor. The success may depend on pump and motor design. For example there may be a minimum drive level where the motor stops rotating and just draws current which you want to avoid.
You may be better off just doing an adjustable timer where the pump runs full for so many seconds then shuts down for so many seconds.
Running the pump slower may just result in the first third of your chest getting cooled. Running it at full speed for a time that flushed all the tubing in a given area may be better and use some amount of therma insulation to avoid an extreme temp shock allowing the cool water to gradually transfer heat from the body. After some time, when the tubing 'charge' has depleted its absorption of heat, then reflush the tubing with a new charge of cool water.
NASA uses this water cooling scheme in their space suits. You might do some google quiries to see what turns up on their work.