We must bear in mind that John is describing the vision God gave him while he was on the island of Patmos for the sake of Jesus Christ. Those events that make up the vision are still in the future, and we should watch for them, for then we will know....
Book Of Revelation, Second Angel, Blood Of A Dead Man
When the second trumpet sounded, what appeared to be a great mountain, burning with fire, was cast into the sea so that a third of it became blood, and a third of the creatures in the sea were killed, and a third of the ships were destroyed.
Revelation Trumpets, Second Angel, Second Trumpet, Sea Became Blood