Full Idea
There is a disanalogy between 'necessarily water=H2O' and 'necessarily Hesperus=Phosphorus'. The second just needs the necessity of identity, but the first needs 'x is a water sample' and 'x is an H2O' sample to coincide in all possible worlds.
Gist of Idea
Unlike Hesperus=Phosophorus, water=H2O needs further premisses before it is necessary
Source
Penelope Mackie (How Things Might Have Been [2006], 10.1.)
Book Reference
Mackie,Penelope: 'How Things Might Have Been' [OUP 2006], p.198
A Reaction
This comment is mainly aimed at Kripke, who bases his essentialism on identities, rather than at Putnam.