1 John 5:6 This is He who came by water and blood—Jesus Christ; not only by water, but by water and blood. And it is the Spirit who bears witness, because the Spirit is truth. 7 For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one. 8 And there are three that bear witness on earth:[b] the Spirit, the water, and the blood; and these three agree as one.

To paraphrase what I posted below, the water and the blood are referencing Jesus’ birth and death. Proof that our savior walked the earth, born of man, and died for us here on earth. They are a witness to His pure, Holy and selfless act.

I found this snippet here:

http://fbctopeka.publishpath.com/the-three-witnesses-the-water-the-blood-and-the-spirit

In 1 John 5:6-12, John tells us that there are three witnesses that deliver the definitive testimony regarding the person and the work of Jesus Christ: the water, the blood, and the Spirit. The “water” (vv. 6, 8) here is most likely referring to the physical birth and life of Jesus, the Son of God (see John 3:5-6, where Jesus equates being born of water to being born of the flesh, as compared with being born of the Spirit, using a form of parallelism). It could also be grouped together with “blood” in referring to Christ’s death on the cross, specifically the moment where the spear pierced Jesus’ side (in John 19:34-35 ESV, John writes, “But one of the soldiers pierced his side with a spear, and at once there came out blood and water. He who saw it has borne witness–his testimony is true, and he knows that he is telling the truth–that you also may believe.”). “Blood” (vv. 6, 8) clearly points us to Jesus’ physical, sacrificial death on the cross for us.

John’s use of these terms is very intentional–Jesus, the Son of God, was indeed a physical man. He was physically born in the flesh, and physically died. These facts are essential to the gospel message. If Jesus didn’t physical live and die for us as the perfect sacrificial Lamb, then we could not be redeemed and restored to God, because we would still be under the wrath of God.

John then says that the Spirit testifies, too. The Spirit testifies within the life of the believer, because God’s Spirit now lives within us. The verb form here signifies a continual action: the Spirit continually speaks to us and leads us in the way of truth, and points us to Jesus as our only hope of salvation