Hello,
I have seen Canada geese do this on several occasions, when stressed and
following a rowdy bath period.
Also, trumpeter swans do this as well, swimming up to 50-75 feet
underwater. They can even turn underwater. I have seen swans do this when
they are finishing a vigorous bath and also when it appeared they were
playing - running up and down the pond then suddenly diving and swimming
just below the surface.

Martha Jordan
marthaj at premier1.net