Atoms are really small.

Consider the number of water molecules in a 12 ounce glass of water, compared to the number of water molecules in all of the oceans combined:

12 ounces of water = 355ml = 355g = 19.7 mol =~ 10^24 molecules

The volume of the ocean is 1.335 * 10^24 ml = 1.335 * 10^24 g water = 7.4 * 10^22 mol =~ 10^45 molecules of water in the ocean.

Let's assume the water molecules in our glass are special, and are the color red, so we can keep track of them. We dump the glass into the ocean and stir it up such that the red molecules are evenly distributed:

The fraction of molecules in the ocean that are colored red = 10^24 / 10^45 = 10*^-21.

Thus, if we were to take a new 12 oz glass of water from the ocean, we would have roughly 10^-21 * 10^24 = 1000 red water molecules in our glass.

Have a nice day!