The Greek text of John 3:5 reads as follows,
"ἀπεκρίθη Ἰησοῦς, Ἀμὴν ἀμὴν λέγω σοι, ἐὰν μή τις γεννηθῇ ἐξ ὕδατος καὶ πνεύματος, οὐ δύναται εἰσελθεῖν εἰς τὴν βασιλείαν τοῦ θεοῦ."
Which is literally translated, "Answered Jesus, 'Truly, truly, I say to you, unless someone is born of water and Spirit, he is not able to enter into the kingdom of God'."
And which most popular translations render as, "Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God." 
One of the burning questions in this text has been, what does ὕδατος here refer too? That the Catholic fathers are in agreement with the traditional and apostolic interpretation of hudatos as water baptism are the following examples:
· The Shepard of Hermas, "to come up through the water, so that they might be made alive; for they could not otherwise enter into the kingdom of God (John 3:5), except by putting away the mortailty of their former life...they go down into the water dead, and come out alive." (9, 16, 2).
· Tertullian agrees and writes, "Since it is in fact prescribed that no one can attain to salvation without Baptism, especially in view of the declaration of the Lord, who says: "Unless a man shall be born of water, he shall not have life." (De baptismo. 12. 1) 
· St. Cyril of Jerusalem is in accord with the ancient tradition, "And He (Jesus) says, "Unless a man be born again" - and He adds the words, "of water and of the Spirit, - he cannot enter into the kingdom of God." He that is baptized with water, but is not found worthy of the Spirit, does not receive the grace in perfection." (Catecheses. 3, 4)
· St. Basil is in accord, "This then is what it means to be born again of water and Spirit: just as our dying is effected in the water, our living is wrought through the Spirit. In three immersions and in an equal number of invocations the great mystery of baptism is completed in such a way that the type of death may be shown figuratively." (De Spiritu Sancto. 15, 35)
· St. Ambrose of Milan is clear, "Nor on the other hand is there any mystery of regeneration without water: for "unless a man be born again of water and the Spirit, he cannot enter the knigdom of God." Even a catechumen believes of the Lord Jesus, by which also he is signed; but unless he be baptized...he cannot receive the remission of sins nor be recipient of the gift of spiritual grace." (De mysteriis. 4, 20)
· St. Augustine, "It is this one Spirit who makes it possible for an infant to be regenerated through the agency of another's will when that infant is brought to Baptism..."Unless a man be born again of water and the Holy Spirit." The water, therefore, manifesting exteriorly the sacrament of grace, and the Spirit effecting interiorly the benefit of grace, both regenerate in one Christ that man who was generated in one Adam." (Letter to Boniface. 98, 2)
Thus it is quite clear what the regula fidei is concerning this text in John. The fathers equate the regeneration of man by the Holy Spirit at the time of baptism, so they see no roadblocks whatsoever in our text. In modern academic circles this traditional interpretation has been vigorously defended  but objectors are not hard to find.  However, the great Protestant exegete F.F Bruce makes a telling admission, claiming that those who reject the recieved interpretation do so on theological grounds, namley the denial of baptismal regeneration. 
. (ESV). The RSV(C)atholic(E)dition is the same as the ESV. The construction is different in the NRSV but the idea is essentially the same, "Jesus answered, "Very truly, I tell you, no one can enter the kingdom of God without being born of water and Spirit."
. Written during Tertullian's orthodox period.
. By both Catholic scholarship; R.E Brown (John: Anchor Bible. 139-141), as well as Protestant scholars; Luther (22nd and 23rd Sermons on John), H. Ridderbos (Ad loc.), G.R Beasley-Murray (John: Word Bible Commentary. Ad loc). D.A Carson while not holding this view admits that the word water here, "is understood by the majority of contemporary commentators to refer to Christian baptism" (John: Pillar Commentary. 192). Still other scholars have argued that not Christian baptism but rather "John's baptism" B.F Westcott (John. 108-109), or using Jewish proselyte baptism as a earthly contrast to the Spirit's baptism, C.S Keener (John. 548-49).
. Such as Morris in the NICNT, Carson in the Pillar entry and Kostenberger in the BECNT to name but a few.
. Found in Bruce's commentary on John, ad loc.