By God’s ordinance, it is a given of life that creatures must drink (water) to survive. That is why rivers and springs are so vital to a comfortable life (Genesis 21:25; Judges 1:15). But by turning all water in rivers and springs (and sea) into blood, murderers of martyrs have no options left but to drink blood themselves to survive (Revelation 16:6).
4 The third angel poured out his bowl into the rivers and the springs of water, and they became blood.