For training purposes it may work to attach a propeller based speedometer to the swimmer's body, though the very disturbed flow around the body will probably mess up your data big time.
Sonar may be another option, from one end of the lane the swimmer is in, measure the distance (I think it'll work best for swimming towards the sensor as the head doesn't move so much), and that combined with time gives you the speed. Then transmit that wirelessly to the goggles.