Possibly, highschool physics has led people astray before =)http://www.google.co.nz/search?num=100&hl=en&safe=off&q=%22resonant+frequency+of+water%22&meta=http://www.physicsforums.com/archive/index.php/t-94766.html
"the microwave oven waves are 2450 MHz, and water has a dipole moment (negative on oxygen, slightly positive on hydrogen side), and when exposed to this electric field the water molecule tries to move to that field, but bumps into another water molecule, thus creating heat. This is not the resonant frequency of water, and the peak absorption of waves decreases as the temperature goes up because of the dielectric properties of water."
But on the other hand...http://www.madsci.org/posts/archives/2004-11/1100632107.Ph.r.html
"The natural frequency of water is a bit more complicated, because it takes into account the mass of water molecules, the attraction between molecules, the distance between molecules, and some other stuff. Suffice it to say that most microwave ovens put out a frequency of 2.5 gigahertz.
... This isn't the lowest (also known as "primary") resonant frequency for water, but microwave manufacturers use 2.5 GHz because they want the microwave to work at any and all water temperatures. There's lots more techno-babble about resonance, matching, and the engineering of microwaves, but that'll have to be a separate question."
Perhaps what you are trying to say is that it is not the fundemental frequency, but a harmonic =). Either way. 2.5GHz is still *a* resonant frequency of water?