Answer
$\approx10^{16}m^{3}$
Work Step by Step
Order of magnitude estimate of the volume of water contained in earth's oceans . Surface area of earth is about $10^{1}.(10^{6}m)^{2}=10^{13}m^{2}$. Thus, the volume of water in the oceans is about area X depth
= $10^{13}\times 4\times10^{3}=4\times10^{16}m^{3}\approx10^{16}m^{3}$
