The road on which this conversation took place crossed several stream beds that empty water from the higher elevations into the Mediterranean Sea during the wetter months. Even though the land generally was desert, water was not entirely absent at some times of the year. The Ethiopian may have already known about water baptism since he had an interest in Judaism. The Jews required water baptism of Gentile converts. Philip may have instructed him further on the importance of baptism (cf. Acts 2:38; Acts 8:12). In any case the official was eager to submit to it. The Jews did not baptize physical eunuchs and take them in as proselytes of Judaism ( Deuteronomy 23:1). If he was a physical eunuch, perhaps this is why the official asked if there was some reason he could not undergo baptism as a Christian. [source][source][source]
Obviously there was enough water for Philip to immerse the Ethiopian, the normal method of baptism in Judaism and early Christianity. Some interpreters have argued, however, that the two men may have stood in the water while Philip poured water over or sprinkled the Ethiopian. This is a possible but, I think, it is improbable. The normal meaning of the Greek word baptizo (to baptize) is to immerse, and this was the common custom. The Ethiopian official testified to his faith in Jesus as the Messiah by submitting to water baptism (cf. Acts 2:38; Acts 8:12). [source][source][source]