Objection 1. It would seem that Penance is not a second plank after shipwreck. Because on Isaiah 3:9, "They have proclaimed abroad their sin as Sodom," a gloss says: "The second plank after shipwreck is to hide one's sins." Now Penance does not hide sins, but reveals them. Therefore Penance is not a second plank.
Objection 2. Further, in a building the foundation takes the first, not the second place. Now in the spiritual edifice, Penance is the foundation, according to Hebrews 6:1: "Not laying again the foundation of Penance from dead works"; wherefore it precedes even Baptism, according to Acts 2:38: "Do penance, and be baptized every one of you." Therefore Penance should not be called a second plank.
Objection 3. Further, all the sacraments are planks, i.e. helps against sin. Now Penance holds, not the second but the fourth, place among the sacraments, as is clear from what has been said above (65, 1,2). Therefore Penance should not be called a second plank after shipwreck.
On the contrary, Jerome says (Ep. cxxx) that "Penance is a second plank after shipwreck."
I answer that, That which is of itself precedes naturally that which is accidental, as substance precedes accident. Now some sacraments are, of themselves, ordained to man's salvation, e.g. Baptism, which is the spiritual birth, Confirmation which is the spiritual growth, the Eucharist which is the spiritual food; whereas Penance is ordained to man's salvation accidentally as it were, and on something being supposed, viz. sin: for unless man were to sin actually, he would not stand in need of Penance and yet he would need Baptism, Confirmation, and the Eucharist; even as in the life of the body, man would need no medical treatment, unless he were ill, and yet life, birth, growth, and food are, of themselves, necessary to man.
Consequently Penance holds the second place with regard to the state of integrity which is bestowed and safeguarded by the aforesaid sacraments, so that it is called metaphorically "a second plank after shipwreck." For just as the first help for those who cross the sea is to be safeguarded in a whole ship, while the second help when the ship is wrecked, is to cling to a plank; so too the first help in this life's ocean is that man safeguard his integrity, while the second help is, if he lose his integrity through sin, that he regain it by means of Penance.
Reply to Objection 1. To hide one's sins may happen in two ways: first, in the very act of sinning. Now it is worse to sin in public than in private, both because a public sinner seems to sin more from contempt, and because by sinning he gives scandal to others. Consequently in sin it is a kind of remedy to sin secretly, and it is in this sense that the gloss says that "to hide one's sins is a second plank after shipwreck"; not that it takes away sin, as Penance does, but because it makes the sin less grievous. Secondly, one hides one's sin previously committed, by neglecting to confess it: this is opposed to Penance, and to hide one's sins thus is not a second plank, but is the reverse, since it is written (Proverbs 28:13): "He that hideth his sins shall not prosper."
Reply to Objection 2. Penance cannot be called the foundation of the spiritual edifice simply, i.e. in the first building thereof; but it is the foundation in the second building which is accomplished by destroying sin, because man, on his return to God, needs Penance first. However, the Apostle is speaking there of the foundation of spiritual doctrine. Moreover, the penance which precedes Baptism is not the sacrament of Penance.
Reply to Objection 3. The three sacraments which precede Penance refer to the ship in its integrity, i.e. to man's state of integrity, with regard to which Penance is called a second plank.
Source: Summa Theologica (New Advent)