William Burkitt on Revelation 16:3

Observe here, That this second and third plague poured out upon the sea, and the rivers of waters, whereby they became blood, have a manifest allusion to that plague of Egypt whereby all the waters were turned onto blood, Exod 7:19.

Observe farther, That by the sea and the rivers full of blood, the bloody persecutors in the antichristian church are supposed to be meant, who by slaughter and bloodshed seek to plant and propagate a bloody religion throughout the earth.

Observe lastly, That the pouring out of these vials gradually and successively upon idolaters and persecutors, and not altogether and all at once, but one after another, teaches us how abounding the patience, forbearance, and long-suffering goodness of God is, even towards the worst of sinners; not willing that any should perish, but that all should come to repentance.