Google Earth just released version 5.0. The latest release has a realistic ocean. In the older versions, flying over the ocean meant mostly that you looked at a lot of blue texture with the occasional island. In Google Earth 5, you now see this realistically shaded ocean with actual depth. You can even fly below the surface to see the ocean floor beneath.
Mars is also now in Google Earth. You can toggle between Google Earth, Sky, and Mars. You can also toggle through time to see historical satellite imagery back to 1991. You could really get lost just exploring, and the educational opportunities are staggering.
I did find an interesting glitch with the historical imagery. I looked at an area I knew had done a lot of construction recently. The most recent image, labeled 2009, is actually several years old and shows several now completed buildings as still under construction. The 2007 imagery shows the completed buildings. I'm guessing the less recent images were higher resolution, but I also have to wonder how many other people will notice similar discrepancies with the history feature.
History glitch aside, this is definitely worth upgrading your current version. You can download Google Earth 5 at http://earth.google.com.