[TXnMA]

Fifteen cubits upward did the waters prevail;

The "cubit", typically used to calculate the size of Noah's Ark is 18 inches -- or 1.5 feet.

A fifteen cubit flood would be 22.5 feet deep.

That is directly from Genesis.

[aruanan]

A fifteen cubit flood would be 22.5 feet deep.

That was 15 cubits above the highest landforms, not a mean depth.

[Secret Agent Man]

If that was the only sentence you’d be right. Taking it by itself is taking it out of context.

The context is that the flood waters rose 15 cubits higher that the highest mountain on earth. It has already been established in Genesis that the flood waters covered the whole earth before this statement. this statement just provides EXTRA information by telling you how high the water actually was, and it was 15 cubits HIGHER than the highest mountain.