Strictly speaking, the words “through water” are not necessary to complete the thought of salvation by means of the ark; that is, the reference to water is implicit in the word ark.
That Peter nevertheless includes the words indicates that there’s something about the water we need to note. What is it? The water of the flood meant death for all outside the ark. The water of the Red Sea also meant death for the Egyptians when its walls fell on their army (Exodus 14:28). In that sense, water is deadly. In the present passage, Peter reminds his readers that God brought Noah through dangerous water to the other side of the flood, with the ark as his instrument. The Lord finds his way to bring Peter’s readers through their suffering to the other side. This again is encouragement for Peter’s readers.
20 because they formerly did not obey, when God’s patience waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water.