I can't answer that one, Chris. It baffles me as well. I know what "semiliquid" means, and we can look up the word to confirm that it means a substance that is somewhere between liquid and solid -- the conclusion being that it is a thick, gel-like fluid. But that lies in complete contradiction to the next two words in his sentence: "thin" and "watery". So maybe on a degree of liquidity, it was only slightly thicker than water.
I'll venture a guess as to why he wrote that -- and that's all it is. Anyone's guess is just as good as mine.
The natural fluid inside the vaginal vault being a somewhat thick mucous, if mixed with blood would still be a rather thick fluid. Both being thick, they would not mix well and (I would think) would instead be kind of streaked between the clear mucous and the opaque red blood. (Have you ever blown your nose and looked at the Kleenex to see blood streaked through the nasal mucous?) But if you add water to this, the water would dilute both so they could more easily blend together with the water and become thin and watery in comparison to their natural viscosities. I doubt Dr. Meyer thought this all out when he observed it -- he was simply describing what he saw. And what he saw was a fluid that was thinner in viscosity than blood or mucous, but not quite as thin as water. Nor was this fluid as deep a color as the red in blood. This could be another reason he described it as watery -- to indicate the degree of deepness in color. All-in-all, it is not a very good choice of words for the description, but nevertheless it conveys (I think) what was present inside her that was not normal. When he wrote it, he fully expected he would be able to explain exactly what he meant in court. (Too bad he never got the chance.)