If all humans descend from Noah, when did the first humans appear in the Americas and how did they cross the ocean to get there?
The evidence from multiple angles, most importantly from C-14 data, but also from stratigraphic, the disappearance of megafauna and other kinds of evidence is that humans entered the Americas around 15,000 years ago. The evidence for this is quite strong. Evidence for humans in parts of North America before this time, perhaps as early as 40,000 years ago, has been presented, but it is not conclusive. For the sake of argument, let us accept the age of the first human settlement in the Americas as at least by 15,000 years ago.
So, how does this relate to the flood? There are two possible answers. First, we do not know the date of the flood of Noah. Before Genesis 12, dates in the Genesis narrative are difficult or impossible to establish. Jewish genealogies are not complete. We know this from many examples. The lengths of time may or may not be literal. I do not know. What we know from the Bible is that some time in the fairly distant past a great world-wide flood happened. Could it have been before the Americas were populated? Yes, that is certainly possible. This would explain both the biblical and the scientific data.
Another possible explanation is that there was, as reported in Genesis 7-9, a great world-wide flood, but that this flood did not literally wipe out all life. Perhaps koalas survived in Australia and llamas in South America. Perhaps there were remnant human populations after the flood as well. There are records of a great flood in cultures as widespread as Australian Aborigenes, North American native cultures, China, India, African and Near Eastern cultures. Might these records be from the flood of Noah? Yes, they might, but that would mean that there were survivors in various parts of the world.
If the second explanation is correct, and I do not know if it is, then the flood may have happened after humans reached the Americas. Sorry I cannot give you an absolutely solid answer, but must leave you with a couple of options. What I am confident of is that the Bible is inspired by God, that there was a great flood, that it was about God’s judgment on human sin. The exact date and exactly how it relates to humans in North and South America I cannot say for sure.