With the latest release of Google Earth, yes the version number has also changed to 5.0, there are a lot of changes and additions which adds more value and quality to the already superb globe browsing experience Google Earth provides and the most important addition is the ability to go undersea.

google earth

Yes the ocean can now be explored with tons of informational media and in 3D as well. From the official Google blog

We were joined at the Academy by many of the dozens of ocean scientists and advocates who helped make this project a reality: friends from National Geographic, the Monterey Bay Aquarium, the National Oceanic and Atmospheric Association, the US Navy, Scripps Oceanography, and Woods Hole Oceanographic Institution, to name just a few.

Edit: Update (Found the video from LifeHacker)

Download Google earth from http://earth.google.com which will download Google updater and will update your earth installation to latest one or if you have problem installing through Google updater try downloading Earth installer from http://dl.google.com/earth/client/current/GoogleEarthWin.exe and update through it.