I don't know of any symbolic importance of this, beyond the obvious: this is very unusual! Blood is blood, water is water, and they don't show up together inside the human body. When was the last time you saw someone bleed like that?
What's interesting is that such a condition is actually quite consistent with modern medical knowledge, under a very specific set of circumstances: this sounds a lot like a cardiac rupture. Intense and prolonged emotional and physical stress can cause the heart to rupture, which brings a very fast death and causes other fluids around the heart to mix with the blood, which are much thinner and less colorful. If the soldier's spear went in at the right place, it could have let this mixture out, which would have appeared to be "blood mixed with water" to an observer.
This is not how a person being crucified usually died, by the way. Crucifixion was a long, slow torment that eventually ended in gradual death by exhaustion. Someone who was only a few hours in crying out with a loud voice and then suddenly expiring was quite out of the ordinary! John's point here seems to be that, while Jesus died on the cross, he did not die of the cross. Instead, as he had claimed earlier on in his ministry, he had life in himself, and he had the power to lay it down and to take it up again, and no one would take his life from him.