@Herald,
Just because you asked a question Im going to ask you to do the calculations (since you are proud of your mathematical prowess)> Therefore Im going to let you do the math.
From the Water Encyclopedia ed 3 , the total volume of water on the planet is 1,385,000,000 km^3. The TOTAL volume of the planet is 1,080,000,000,000 km^3.
1What percent of our planet's total volume is water?
2 What would the dimensions of a single comet be that could deliver such an amount of water? (SEE BELOW)
3 Assuming that all water accretion took place after the HAdean period, how frequently would comets of say 0.0001% of that amount have to strike the earth to provide "seed" water?.
Time and volume, that's all. You can use the mensuration formula for the volume of a sphere (in km^3)
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If
V= 4/3 X (3.14) X r^3 (in km)
and solve for r (You can then multiply r X 2 and that will provide you the approximate diameter of the space body (Lets assume its a sphere, even though this is probably not the way these things looked)