It's not really an island, but an alien lab/observation post. The power source used to maintain the post's stability in the water has been leaking, which is what caused the plane to crash and some of the "miraculous" recoveries, such as with Locke and potentially Jack's dad (what was up with the empty coffin, anyway?).
As a result, the "island" is destabilizing and is starting to sink slightly; so what they think is the tide rising is really the island sinking.
Okay, maybe that's a little far-fetched as well...