As this "MOUNTAIN" is to fall on the sea, and the Judgments poured forth by the "Trumpets" and "Vials" are to fall mainly on that part of the world bordering on the Mediterranean Sea, it is highly probable that the "SEA" here mentioned is the Mediterranean Sea.
Notice that John does not say that it was a "Mountain" that he saw cast into the sea, but that it appeared like a mountain, not a burning volcano, but an immense meteoric mass ablaze with fire. That was as near as John could describe it. The effect of this "burning mass" on the sea, into which it fell, was to turn a third part of the sea into BLOOD. If any are disposed to doubt the possibility of such a thing let them turn to the "First Egyptian Plague" Ex. 7:19-21, and read the account of the turning of the waters of the River Nile into BLOOD. "And all the waters that were in the river were turned to BLOOD. And the fish that was in the river DIED; and the river STANK, and the Egyptians could not drink of the water of the river: and there was BLOOD throughout ALL THE LAND OF EGYPT." The only difference between the "First Egyptian Plague," and the effect on the Mediterranean Sea by the falling of a "Meteor" into it at the sounding of the Second Trumpet, will be, that only 1/3 of the sea shall become BLOOD, that where the "Meteor" strikes, and thus only 1/3 of the living creatures in the sea shall DIE. One-third also of the ships shall be DESTROYED, possibly by a tidal wave, or the "Meteor" may fall on a fleet of naval vessels, like the storm that destroyed the Spanish Armada.