Panoramio recently made an announcement that should be of interest teachers using Google Earth in their classrooms. Panoramio is now accepting underwater imagery. The new underwater imagery contributed to Panoramio should enhance the underwater layers of Google Earth. If you've never explored the underwater imagery of Google Earth, the video below offers a short introduction.
Here are some related items that may be of interest to you:
Google Earth Across the Curriculum
Historic Maps in K-12 Classrooms
History Pin - Historical Images in Google Street View