I can't imagine a water park goes with the whole marine reserve theme. Also, I would not think they would close the island to build something in the middle of the island that is pretty much closed off now. The more I think about it, it most likely is to widen and deepen the channel and maybe extend the docking area. This will also make docking in higher seas easier for the other ships and less missed spots.
Other work on the island can then be done during this time, like making the Ocean House bigger. They might even address a lasting fix for the YC rock issue