Your question comes down to whether the EM absorption is a resonant process or not, where resonant means it corresponds to the energy of some excitation of the water molecule. The answer is that it is not a resonant process. Microwave ovens operate at 2.45GHz but the lowest energy transitions of water molecules are rotational transitions, which have energies in the 100GHz to 1THz range. The energy of the photons in a microwave oven are too low for any resonant absorption.
Google for details of the rotational spectrum of water. I found examples here and here.
The EM radiation from the oven makes dipolar molecules within it line up with the electric field. As the field oscillates the water molecules change direction (at 2.45GHz). In liquid water the molecules interact strongly and exchange energy with each other, so the energy of the flipping motion gets transferred to translational energy of the water molecules i.e. heat. Because this is not a resonant process changing the microwave energy by small amounts (up to an order of magnitude) won't make a lot of difference to the heating. As some of the comments have mentioned, this process is called dielectric heating.