This video says about itself:
The team use a very special underwater camera housing called the megadome which enables them to film in super wide-angle, allowing the audience to see both above and below water simultaneously. With a fully grown mother walrus to contend with, David Reichart the cameraman must take special care not to disturb his subject or risk a tusk in the head!
Scientists have uncovered a 10 million year old walrus fossil in Japan. Despite lacking the tusks that modern walruses are famous for, the fossil is still part of the same family as present-day walruses: here.