Number of water molecules to contribute one molecule to every milliliter on earth
A very quick Googling tells me that the volume of the oceans is 1.3 billion $\mathrm{km}^3 = 1.3\times10^9\;\mathrm{km}^3 = 1.3\times10^{18}\;\mathrm{m}^3 = 1.3\times10^{24}\;\mathrm{cm}^3 = 1.3\times10^{24}\;\mathrm{mL}$.

To get one molecule into each of those mL, we'd need $1.3\times10^{24}$ molecules of water, or $\frac{1.3\times10^{24}}{N_A} = \frac{1.3\times10^{24}}{6.02\times 10^{23}}$ moles of water, about 2.2 mols.  Since water is 18 g/mol, that comes out to 39 grams of water or about 39 mL.