For the most part, the two separate strands of the Flood story are relatively easy to distinguish from each other. Most of the time, J and P's writings are vastly different in style and in concept; but in some places the two seem to share a common style branching from a very different concept.
J's strand is more informative in an entertaining way than P's strand. J starts off chapter 6 by telling of the wickedness of the Nephilim, "The sons of God came into the daughters of men, and they bore children to them" (Gen. 6:4). J then goes on to say that God thinks everyone is evil, and He sorry that He created people and will now destroy them. P simply states that "The earth was corrupt in God's sight, and the earth was filled with violence" (Gen. 6:11). Then P also goes on to say that God is sorry that He created people and will now destroy them. While both came to the same conclusion, J's strand was more specific and entertaining than the direct, no frills style of P.
On the other hand, when P thinks details are necessary for authenticity's sake, rather than, for entertainment; P can be quite verbose. P writes that God tells Noah: "I have determined to make an end of all flesh; for the earth is filled with violence through them; behold, I will destroy them with the earth. Make yourself an ark…" (Gen. 6:13-21). There are several other places throughout the Flood story that God is quoted, but the P passages contain the most quotations. It seems to be a very important concept to P. To J, what happens to Noah and his family are more important than the actual words of God. "And Noah and his sons and his wife and his sons' wives with him went into the ark," (Gen. 7:7).
J tells a story of human survival. P tells a story of God's deeds.