Genesis7:20

The options are that God told Noah, Noah could have taken depth soundings, or Noah made calculations based on what he observed.

It is possible that God told Noah that the depth of the water was more than what was needed to cover any giant on the top of the tallest hill. 15 of the early cubits would have been about 25 feet today. This does not prove they had giants of 25 feet tall; it could mean it was enough water to cover him with several feet of water, or that 25 feet was enough to cover him with water twice as deep as he was tall. The twice his height option fits well with the way God does things. The point was that no one survived outside the ark.

A moon pool for ventilation could have been used by Noah to take depth soundings; while throwing something out of the small air gaps at the top of the ark would have been close to impossible. In fact, they could have built into the ark, the cables, pullies and other things they would need to take such soundings from the moon pool. But the only thing that soundings would do is show how deep the water was under their location, which constantly changed. Maybe the statement about 15 cubits (Gen 7:20) simply shows the smallest sounding they recorded (thus the highest point of land) after the water covered everything.

It is also possible that Noah was close to the highest hill in his area, which he could see by peeking out of the air vents at the top of the ark. He was able to note how quickly the water was rising toward the top of the highest hill. He probably calculated that increase in cubits per day.   After the top of the hill was surpassed, he kept up his calculations until something felt different. There was probably a pause which came when the level of the water stopped rising. Noah did not sense that the water was no longer rising, but he probably felt the pause, possibly as a lessening in the frequency and harshness of the waves.

We can be sure that when the waters overtopped the highest hills, the last surviving humans would have been swept off of their high perches by tsunamis which relentlessly reached higher and higher. It would have been useless to try to survive by hanging on to floating logs, or by building a small raft. The waves and hypercane conditions made all such efforts futile.