We need to consider more of the passage, I think, to understand what the word ἀντίτυπος means in context:
1 Peter 3:20–21 (NIV)
... to those who were disobedient long ago when God waited patiently in the days of Noah while the ark was being built. In it only a few
people, eight in all, were saved through water, and this water
symbolizes baptism that now saves you also ...
The English word "antitype", though somewhat obscure, would have been the literal translation here of ἀντίτυπος - antitypos. A more literal translation is found in the Orthodox New Testament:
... who once were disobedient, when the long-suffering of God was waiting in [the] days of Noah, while an ark [was] being constructed,
in which a few, that is, eight souls, were saved by means of water.
There is also an antitype which now saveth us - baptism ...
The water of the flood, explains Peter, is an Old Testament type (typos) - a foreshadowing - of the later antitype of baptism.
John Breck, former Professor of New Testament studies at St. Vladimir's Orthodox Seminary, writes of the use of types and antitypes in Scripture in his book, Scripture in Tradition: The Bible and Its Interpretation in the Orthodox Church (p.22):
Within the framework of salvation-history, select events that occur in
the experience of a people, particularly the Hebrew people, constitute
"types" (typoi) or prophetic figures of persons, events and
institutions to come, that will be fulfilled in the messianic age.
The relation between the two Testaments is a "typological"
relationship in which God's promises of salvation, expressed by events
in Israel's history as well as by oracles of the prophets, will be
fulfilled in the person of Jesus Christ and in the life of the Chruch.
The Old and New Testaments represent a unified witness to
salvation-history. The relation between the two Testaments is that of
Promise and Fulfillment. An inner, organic unity exists between the
two, such that key persons and events of the Old Covenant find their
ultimate meaning in those of the New. This relationship of Promise
to Fulfillment, inherent in the historical process itself, can be
described as a relation of type to antitype.