Adam Clarke on Acts 8:38

And they went down - They alighted from the chariot into the water. While Philip was instructing him, and he professed his faith in Christ, he probably plunged himself under the water, as this was the plan which appears to have been generally followed among the Jews in their baptisms; but the person who had received has confession of faith was he to whom the baptism was attributed, as it was administered by his authority.