Plasma is about 90% water. Red blood cells contain water too, "dry content" is what you get by removing it.
By 'dry content' I mean chiefly proteins (Hemoglobin being the most important one; surface glycoproteins which assign us to a particular blood type, membrane proteins - channels, transporters and enzymes), also membrane lipids and ions (sodium, potassium, etc)
The dry mass % of a RBC is about 95-97% made up of Hemoglobin, basically a complex protein containing iron, that transports oxygen to the tissues and collects carbon dioxide back to the lungs.
There are about 280 million molecules of Hemoglobin in one single red blood cell. The fact that whole blood or red blood cells aren't permitted for transfusion, but Hemoglobin based substitutes like PolyHeme (from human Hemoglobin) are okay now, just shows how arbitrary and unscientific the WT stance is.