Fully immersion is the standard for all baptisms. However, due to quite reasonable safety concerns, infants are often immersed in the following manner: they are first placed in the baptismal font and supported with their heads held out of the water. The priest then scoops water to fully annoint the head while intoning "I baptize you in the Name of the Father." The child is then pulled from the font, lifted high and then returned for the blessings to the Son and again for the Holy Spirit.
To answer your question, the head must be fully wet.
Thanks. Unlike y'all Orthodox, I have never been nekkid in church. I trust you make the most of the opportunity. 8-)