Rock of Salvation
While the common metaphorical interpretation to the water are purely opinion based, the attempts to explain the verse with scientific physiological manner is a result of misguided approach that confuses the Midrashic Gospel of John with historical. Ancient Historiographies and history are two very different things. The actual reason behind the verse is John's allusion to the Midrash on the Moses rock of salvation that produced blood and water when Moses hit it with the staff. John's gospel is primarily Midrash, therefore, we see a special emphasis on Jesus being the water of life. It is important to recognise the actual literary and cultural context of the Gospels to determine the intended meaning.
The key passage of the rock is in Exodus 17 and Numbers 20. The midrash is available on Sefaria:
Rabbeinu Bahya, Bamidbar 20:8:8
Water turning into blood was certainly also a manifestation of the attribute of Justice at work....Our sages in the Midrash and the Talmud are on record that originally “the sela dripped blood,” i.e. that when Moses hit the rock it first produced blood instead of water....This may also be the source of Psalms 78,20 speaking of ”here he struck the rock and water flowed.”...The word ויזובו used by the psalmist to describe the flow is one that does not usually describe water flowing but blood flowing ....Psalms 114,8 also suggests that the blood turned into water .
Torah Commentary by Rabbi Bachya ben Asher, trans. Eliyahu Munk, 1998.
Otzar Midrashim, Aharon, Midrash on the Death of Aharon 3
Moses and Aaron gathered the people around a different rock, as it says in the Torah "Moses and Aaron gathered the community at the face of the rock."...This time though, blood came out, as it says in the Psalms, "yes, he hit the rock and the waters flowed ."...It's a flow of blood, as we see from the laws of menstruation "when a woman has a discharge of blood ." Moses came and said to God "the rock isn't bringing out water, only blood."...God said to the rock: "why are you only bringing out blood, and not water?" The rock said "Master of the Universe, why did Moses hit me?" God then asked Moses, "why did you hit the rock?"...Therefore God said to Moses, "tell the rock to turn it's blood into water." It is thus written in scripture: "who turned the rock into a pool of water, the flinty rock into a spring ."