Isotopes of Elements
Most periodic tables quote the molar mass of an element as the average mass of the element given the natural abundance of the isotopes. Chlorine, for example, has a standard atomic weight of 35.45 which is a weighted average of ~75% 35Cl and ~25%  37Cl, the two common isotopes.

But isotopes of hydrogen and oxygen are rare. So the vast majority of water molecules will not contain them. So, at least to one decimal place, most water will have a molar mass of 18.0. 

At it's natural abundance of about 0.25% 18O won't noticeably affect the molecular weight of water until you can measure it to at least 3 decimal places of precision. Slightly simplifying (assuming pure isotopic masses are exact integers, which they are not), water containing that isotope at its natural abundance will have an an average mass of 18.005 rather than 18.000.

When working with normal (unenriched) substances the best assumption is that they will contain the normal isotopic abundance of the component elements (which are well known and, for most elements, don't very much by source). But the effect on molecular mass will often be small and you will need a fair amount of precision to notice it. If you are not working to very precise masses, it is fair to assume natural isotopic abundance.