What prevented the antediluvian humanity from using ships against the Flood, or from using their already existing ships? At least a few of them could have survived the Flood that way.
There are several reasons as pointed out by Rav Sorotzkin, Rav Hirsch and others.
First of all, they could not have stored sufficient food to last for the entire flood. The flood lasted for an entire solar year. In fact, it was only by a miracle that Noach and the animals had sufficient food during the flood and while they were repopulating the Earth.
As an example, Columbus almost did not make it in 1492. Had the land been only a little farther off, his three ships would not have succeeded. Reports that I have read also said that they had passed the point of no return and would probably not have been able to return safely to Spain.
Secondly, the ark only survived the violence of the flood by a miracle. Any other vessels would have been destroyed.
Third, Rav Hirsch points out that the word used in Pasuk 8:21 for all life on the Earth dying, ויגוע, means "became torpid" or in effect died in their sleep. That is, Hashem put everyone to sleep so that they they died peacefully at the beginning of the flood rather than being able to flee to the mountains and die as the water covered them.
Their own stubbornness. (As Monica said in a comment.)
Note similarly in Exodus, Moses warns the Egyptians that anyone/anything left outside will be stricken by hailstones. Some Egyptians are willing to at least consider this possibility, and move their slaves and cattle inside. But a lot of the Egyptians are too proud to even openly acknowledge that they might be wrong, and leave everything alone.
Noah has been turning blue in the face telling the people to improve their ways, and they don't listen. Now he's building a boat, and they continue with business as usual. Then it was too late.