Gravity is the force
Mass is the property
Inertia is the consequence
EG
Gravity interacts with objects with the mass property, and with naked spacetime. The force is conveyed by hypothetical gravitons.
The mass property is what determines how "sticky" the object is to spacetime. It is conveyed via the higgs particle.
Inertia is the consequence of a massed object interacting with spacetime and with some other kind of energy. (space is elastic, yo.)
A water molecule with no mass will not be sticky to spacetime. Being massless, it will travel at C as a necessity. (the same way a photon does.) It has charge, charge-self-interaction, and spin properties. Without mass, the attractions and repulsions of those forces will function more like EM feilds than discrete particles. You will have curious things indeed if you made water massless. It would likely take on some kind of plasma dynamic form. Reducing the mass would make the consequences (inertial products) of those charges and spin properties remaining the same become quite apparent. The situation I painted prior about water taking on unusual fluid and solvent capabilities are consequences of reducing the inertial requirements of the water molecules by meddling with their masses. It takes less energy to dissolve substances all of a sudden, for instance. (Heat of crystallization when in aqueous solution changes, specific heat of the water changes, all kinds of things.)