Water must at least touch the head.
If the priest just dipped the baby, for example, butt first and only the butt/torso got wet this is NOT a valid baptism. You can find info on “butt baptisms” at places like Fr Z’s blog. Fr. Z received confirmation from the CDF that the head MUST touch water.
If the priest dipped the baby such as the back of the head was in water, it is valid. It isn’t necessary to dunk the baby’s face under the water.
It is best if water is poured over the baby’s head so there is no doubt.
You witnessed it so you will have to say if the baby’s head actually was put in the water or not. If not, I suggest you urge the parents to contact the diocese about this matter. Invalid baptism means no other sacraments would be valid. Or simply take the baby in the bathroom and peform a conditional baptism to ensure it is properly baptized. But really, if the priest is doing this improperly it should be brought to his attention or the diocese’s attention to ensure it is corrected.