Assuming you mean what happened to the mixing of the supposedly freshwater rain and the salt oceans, this is a reminder that the bible doesn’t claim the flood came only from rain. The flood water also came from “the fountains of the deep”. We don’t know what the composition of that water was.
In an article in Nature the lead author, a mantle geochemist says “… the transition zone could hold as much water as all the Earth’s oceans put together.“
In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month, on that day all the fountains of the great deep burst forth, and the windows of the heavens were opened.
Gen 7:11 ESV
It has been speculated by YECs((YEC is a common abbreviation for Young Earth Creationist)) that some oceanic ridges are the scars where the mantle was torn allowing the deep water to burst out. There’s also been speculation the rain was caused by the violent release of deep water, jump-starting or boosting the water cycle.
There was a massive earthquake before the flood:
And on that day, the Lord caused the whole earth to shake, and the sun darkened, and the foundations of the world raged, and the whole earth was moved violently, and the lightning flashed, and the thunder roared, and all the fountains in the earth were broken up, such as was not known to the inhabitants before…
Jasher 6:11 (non-canonical but referred to directly in canon)
And a week later, the flood was upon them:
And at the end of seven days, in the six hundredth year of the life of Noah, the waters of the flood were upon the earth.
And all the fountains of the deep were broken up, and the windows of heaven were opened, and the rain was upon the earth forty days and forty nights.
Though the rain stopped after 40 days, the flood lasted 150 days, and it took a year for the water to recede enough to leave the boat and walk on land. All that is in the text, but this is already getting longer than most probably have the patience for.
So how did the marine life make it? Looks like most of them didn’t. 90%+ of all known fossils are invertebrate marine life.