When I initially learned about substitution reactions, there would be times where a poor leaving group had to be changed into a better one in order to proceed with a substitution reaction. For instance, changing a poor leaving group (-OH) to a better one, such as water:
In this case, the water leaves by itself. My question is:
If I ever come across an instance where water (a good leaving group), must leave, how should I assess whether it can leave BY ITSELF or not?