If you are refering to 1 Peter 3:20 (below):

1 Peter 3:20
20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
KJV

This does not say "saved from water" or "saved in the ark" it says that they were saved by water.

Noah's perfect pedigree was saved by the water! The Kenites were already mixed and thusly were not saved by the water.

I am going to have to put this together as I understand it. The writings of Peter require an understanding of God's overall plan.

Genesis 3:15
15 And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
KJV

Genesis 6:9
9 These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God.
KJV

Noah had a perfect pedigree.

Eve's seed shall bruise thy head (Satan's head). This is a reference to Christ, who was to come from the line of Adam.

1 Corinthians 15:45
45 And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit.
KJV

OK, here is what is going on. The world was full of Nephilim (Human/Angel) hybreds. The situation became so bad (the daughters of Adam mixing with non-Adamic hybreds) that it became necessary for God to step in and not only destroy the Nephilim but preserve the only pure blood Adamic family left, Noah, his wife, three sons and their wives. (8 adamic souls).

When the flood came it destroyed the Nephilim thus preventing them from contanimating the genealogy of Noah's family. It was from this family that ultimately Christ would come. There was simply no mention of the other geneologies on board because that was not the subject in 1 Peter 3:20. The subject was Christ becoming a quickening spirit.

This also ties in with the prophecy that Christ would bruise Satan's head. Had the family of Noah been contanimated (Satan's plan in the first place in the Garden of Eden). To prevent the coming of Christ.

Yet the Kenites were to be preserved because of the prophecy “thorns also and thistles shall it bring forth to thee.”

This required Noah to take his family on board and the “beasts that are not clean two by two.” Both prophecies had to be preserved or God’s word would mean nothing.

As a confirmation of what I just posted, let me place one more quote from the Bible, from the book of Luke:

Luke 17:27
27 They did eat, they drank, they married wives, they were given in marriage, until the day that Noe entered into the ark, and the flood came, and destroyed them all.
KJV