This is a rule of thumb offered by my organic professor to guage a molecule as a nucleophile-
*Tend to have an attached hydrogen
Ex. H20, R0H, NH2--R (R= H, C-chain, nitrogen derivatives)
*Tend to have a negative charge
Ex. -OH, -OR, -S
Nucleophilic attack follows Coulomb Laws- opposite charges attract. Nucleophiles, being partially or fully negatively charged, will be atrracted to electrophiles, being partially or fully postively charged. The greater the differences in charges the greater the attraction, which allow for nucleophilic attack. Water, being partially charge, hence, would not want to attack a non-charged carbon. Not enought attraction there... Water, though, can attack a carbon that is fully postively charged. There is enough attraction in this case.