I believe that the ante-Nicene Fathers (as well as the post-Nicenes) generally accepted the historical validity of the Noachian Flood:
"In the ninth generation are born the giants, so called from of old, not dragon footed, as the fables of the Greeks relate, but men of immense bodies, whose bones, of enormous size, are still shown in some places for confirmation. But against these the righteous providence of God brought a flood upon the world, that the earth might be purified from their pollution, and every place might be turned into a sea by the destruction of the wicked. Yet there was then found one righteous man, by name Noah, who, being delivered in an ark with his three sons and their wives, became the colonizer of the world after the subsiding of the waters, with those animals and seeds which he had shut up with him" (1 Clement 29).
In Refutation of all Heresies 26, Hippolytus also
"And in Noah's time there occurred a flood throughout the entire world, which neither Egyptians, nor Chaldeans, nor Greeks recollect; for the inundations which took place in the age of Ogyges and Deucalion prevailed only in the localities where these dwelt. There are, then, in the case of these (patriarchs-that is, from Noah to Heber inclusive)-5 generations, and 495 years."
Moreover, in a work of Theophilus, we read:
"But Moses, our prophet and the servant of God, in giving an account of the genesis of the world, related in what manner the flood came upon the earth, telling us, besides, how the details of the flood came about, and relating no fable of Pyrrha nor of Deucalion or Clymenus; nor, forsooth, that only the plains were submerged, and that those only who escaped to the mountains were saved" (To Autolycus 3.18-19).
The quotes that illustrate the beliefs of the ante-Nicenes regarding the Noachian Flood are many. It appears safe to say that the Fathers generally thought it was a historical event. However, this does not mean that the account could not be read on other exegetical levels. For instance, Cyprian argues that anyone who does not have the church for his mother cannot have God for his Father. He then uses the account of Noah to explain the salvific or soteriological role of the church in the outworking of God's eternal purpose through Christ Jesus:
"If any one could escape who was outside the ark of Noah, then he also may escape who shall be outside of the Church" (On the Unity of the Church 6).
Augustine of Hippo later employs the biblical narrative of Noah and applies it to the church. He likens the clean and unclean animals taken into the Ark to defiled and undefiled persons in the Christian ecclesia. For Augustine, the church is a corpus permixtum: a body composed of different types of persons until Christ does the separating work. However, Augustine thought the Flood was an historical event as did the ante-Nicenes. Interestingly, Origen's interlocutor Celsus, also charged Christians during his time with distorting Greek flood stories by using the biblical account of Noah.