1 Peter 3:21

Noah's salvation in the ark upon the water prefigured the salvation of all good Christians in the church by baptism; that temporal salvation by the ark was a type, the antitype whereunto is the eternal salvation of believers by baptism, to prevent mistakes about which the apostle, I. Declares what he means by saving baptism; not the outward ceremony of washing with water, which, in itself, does no more than put away the filth of the flesh, but it is that baptism wherein there is a faithful answer or restipulation of a resolved good conscience, engaging to believe in, and be entirely devoted to, God, the Father, Son, and Holy Ghost, renouncing at the same time the flesh, the world, and the devil. The baptismal covenant, made and kept, will certainly save us. Washing is the visible sign; this is the thing signified. II. The apostle shows that the efficacy of baptism to salvation depends not upon the work done, but upon the resurrection of Christ, which supposes his death, and is the foundation of our faith and hope, to which we are rendered conformable by dying to sin, and rising again to holiness and newness of life. Learn, 1. the sacrament of baptism, rightly received, is a means and a pledge of salvation. Baptism now saveth us. God is pleased to convey his blessings to us in and by his ordinances, Act 2:38; Act 22:16. 2.

— Matthew Henry's Concise Commentary (public domain)

whereunto--The oldest manuscripts read, "which": literally, "which (namely, water, in general; being) the antitype (of the water of the flood) is now saving (the salvation being not yet fully realized by us, compare Co1 10:1-2, Co1 10:5; Jde 1:5; puts into a state of salvation) us also (two oldest manuscripts read 'you' for 'us': You also, as well as Noah and his party), to wit, baptism." Water saved Noah not of itself, but by sustaining the ark built in faith, resting on God's word: it was to him the sign and mean of a kind of regeneration, of the earth. The flood was for Noah a baptism, as the passage through the Red Sea was for the Israelites; by baptism in the flood he and his family were transferred from the old world to the new: from immediate destruction to lengthened probation; from the companionship of the wicked to communion with God; from the severing of all bonds between the creature and the Creator to the privileges of the covenant: so we by spiritual baptism. As there was a Ham who forfeited the privileges of the covenant, so many now. The antitypical water, namely, baptism, saves you also not of itself, nor the mere material water, but the spiritual thing conjoined with it, repentance and faith, of which it is the sign and seal, as Peter proceeds to explain. Compare the union of the sign and thing signified, Joh 3:5; Eph 5:26; Tit 3:5; Heb 10:22; compare Jo1 5:6.

— Jamieson-Fausset-Brown Commentary (public domain)