It’s not hard to do some calculations like this. I looked up the volume of the Pacific ocean, and the volume of one water molecule, multiplied and took the square root. This gives a geometric mean of 4.6 ml, which is quite close to a teaspoon.
Thus, there are as many molecules in a teaspoon of water as there are teaspoons of water in the Pacific Ocean. If somebody dumped a teaspoon of water into the Pacific, and we waited long enough for it to be well mixed, the average number of those original molecules in a newly sampled teaspoon would be one.