The circumference of the earth is about 24,000 miles, so its radius is about 24,000 / 2_π_ miles ≈ 3,820 miles.
The surface area of the earth is therefore about 4_πr_2 = 183,346,494 miles2.
About 75% of the earth’s surface is covered by oceans; that’s about 137,509,871 miles2.
I don’t know the average depth of oceans, but the deepest part is about 6 miles deep, so let’s say that the average is 1½ miles. Then the volume of ocean water is about 206,264,806 miles3, or about 30,361,757,049,249,400,000 cubic feet, or about 52,465,116,181,103,000,000,000 cubic inches.
Assuming we include the peel on each banana, an average banana is roughly a cylinder 8 inches long and 1½ inches in diameter; its volume would then be about 8 × π(¾)2 = 14 cubic inches.
Therefore, the number of bananas needed to replace the ocean water is about 52,465,116,181,103,000,000,000 ÷ 14 = 3,711,147,813,383,610,000,000.
As to the volume of a banana, I assume we’re talking Cavendish (the main variety) not the miniature ones that you can get. This is where the disagreement is going to come. I’m taking the average banana to be 110 cm^3 or 1.10 x 10^(-13) km^3 as a km is 10^5 cm my answer 1.335 x 10^9 /( 1.10 x 10^(-13)) =1.2136 x 10^22 the original poster had 2.42 x 10^(23)
Gotta love everyone assuming bananas are a perfect cylinder. My answer: one giant, genetically engineered 1.35 billion cubic kilometer banana. Perfectly packed, of course.