During the flood the water was 15 amos (cubits) over the highest mountain. [Bereishis (Genesis) 7:20]. According to one opinion an amah is the equivalent of 18.9 inches (Frank, The Practical Talmud Dictionary). How high above the top of the mountain was the water in feet?
15 x 18.9 inches = 283.5 inches
283.5 inches / 12 = 23.625 feet.