Is the great tribulation earthwide?

YES & yes!!

The book of Revelation is a masterpiece of God and the events that take place which it speaks about are indeed worldwide. Everything from the oceans to all of mankind to even the universe itself is affected by what will occur. Some of its adversaries will say that the events in Revelation already occurred but this is simply not the case and we can use internal evidence to show this. For instance, it says that in the end there will be no more oceans. If the events within the book already took place we would have no more oceans.

Since during it a third of the land a sea is turned to poison by something falling from the sky, yes

That third that is taken out may be North and South America, because they are not mentioned in the end battle

That is what I've always thought, but perhaps worse in some places than in others.