I have never heard of that method.  I don't think it will ever catch on because it would require a change in "how we have always done things."  Besides being a change, it does have one flaw as I see it.  You would only be able to measure antlers that have been removed or with uniform size skull caps.

You couldn't measure for the record horns attached to the skull because the size of the skulll would affect the displacement of the water.  there are probably other variables that would have to be considered and factored in too that I am missing.

But who knows, it might catch on and we'll be saying, "I saw a nice buck last night, I think it went 40 or 50 cubic centimeters...it was a real monster."

40 or 50 cubic centimeters.....

I have never heard of that either, but that is a good idea.  It would be pretty much an exact science, as long as the dunk tank being used is continually kept calibrated.  However, it would seem like alot of work, and as CVC said, it would be a change to the way things are done, so I don't think the general public would go for it.

Sounds like it could be a more thorough way of scoring, but not complete.  Volume takes into account the entire volume.  Length is a big part of that scoring too.  I've never heard of scoring being done this way, but I don't see why it could not be used as a portion of the scoring.  It would have to be adopted as an official method of measure.  I don't trophy hunt so scoring matter little to me.  I'm a meat hunter, so I'm more concerned with size/volume for a different reason.