A Great Mountain is to fall on the sea, and the Judgments that pour from the Trumpets and Vials are to fall mainly on the part of the world bordering the Mediterranean Sea, and it's possible that the Sea mentioned is the Mediterranean Sea.

       John doesn't 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 a huge meteor mass ablaze with fire.

               That was as near as John could describe it. Its effect was to turn a third part of the sea into blood. Much like the Plague in Exodus 7:19,21.