Strong's Concordance tells us that to baptise comes from a word meaning to make fully wet. The word is used concerning ships sinking and with reference to a piece of cloth being dyed. Baptisms recorded in the Bible imply that it is a full immersion.
John the Baptiser chooses a place to baptise where there was much water:
23 John also was baptising in Enon near Salim, because there was much water there. They came, and were baptised;
Mark tells us that John baptises people in the river Jordan:
5 All the country of Judea and all those of Jerusalem went out to him. They were baptised by him in the Jordan river, confessing their sins.
When Jesus is baptised we are told that he went up from the water:
16 Jesus, when he was baptised, went up directly from the water: and behold, the heavens were opened to him. He saw the Spirit of God descending as a dove, and coming on him.
When an Ethiopian official is baptised, the record says that they went down into and came up out of the water:
38 When they came up out of the water, the Spirit of the Lord caught Philip away, and the eunuch didn’t see him any more, for he went on his way rejoicing.
39 But Philip was found at Azotus. Passing through, he preached the Good News to all the cities, until he came to Caesarea.
Related theme: Baptism - its meaning