As a part of the new v3 Google Maps API, Google have announced the addition of a new Elevation Web Service. Limited elevation data has been available from a number of map web services, but the Elevation Web Service allows individual coordinates and complete path profiles to be queried. Going by the samples, it looks like this works for both land and ocean.
I would imagine that this will be useful for hikers and sport teams (eg. the profile of a cycle or running route), as well as geological education ( eg. the ocean profile across a spreading ridge or the Hawaii Plume).
Google’s blog announcement is here.