The island didn't sink because islands don't sink by sprovo

First of all islands are attached to the earth, bottom line. This might be a bad source but this is common knowledge. http://answers.yahoo.com/question/index?qid=20080224092325AAPgC0F

When I first saw the island under water my first thought was that global warming caused the ocean to rise. I just saw it mentioned in another theory and decided to do a bit more research since the first time around it was quickly shot down. At the time the idea of LA also being under water made sense so I moved on. My mistake.

First of all the fact stands that this is already happening throughout the world and here's a quality source two weeks old.
http://news.discovery.com/earth/global-warming-island-nations.html

So if the "island" isn't actually attached to the earth then it's not really an island. My issue with this is the fresh water supply on the island which would suggest to me that it's connected to the earth.

So what did we see from the helicopter when the island vanished?

We saw the immediate effects of time travel although we hadn't witnessed the events of the time travel yet. What we saw was the ocean immediately revert to a reality when global warming has accelerated.

If you go back and watch the scene it's nearly impossible to judge the horizon before and after the flash. Keep in mind that the water nearly instantly fills the void left by the island.

If the island had physically moved I think the water would have rushed in from all directions. That should have caused waves water movement. It didn't.

They moved the island in time backwards which gave the Losties a chance to set off the bomb. That change somehow caused global warming to accelerate and now the island is underwater.

The island could easily be under water while LA stays dry. In that world they would have set up precautions to keep the water out. That's if the water raised to a point that it effected the mainland of a continent which from what I'm seeing wouldn't be the case.

It would take a massive amount of melting in order lose the whole state, even LA. This article suggests that the worst would be seen in San Fransisco.
http://www.nuwireinvestor.com/articles/global-warming-could-claim-100-billion-worth-of-california-real-52704.aspx

Another interesting story on the issue related to the San Fran area.
http://www.foxnews.com/story/0,2933,509037,00.html

The fact that the show has consistently called this place an island leads me to think it is an island attached to the earth. Maybe it can shift along the surface but I think the more accurate idea is that when it's moved it's moved through time.

In that case I think that when the FDW is moved it's supposed to be moved counter clockwise in order to move the island into the future. If the island is moved to the future then the past isn't changed which won't cause changes in time like we're experiencing now. Think of a time line. The FDW going left would suggest going back and that would suggest going right would go forward.

Getting back to the point. When Frank yells "where'd the island go?" in the helicopter I think he was distracted from his instruments. After he gains his composure he says we're going down. Yes they were going down but I think that they had aleady when down from a horizon standpoint since the horizon actually raised. Frank was just amazed by the fact that the island was gone.