The very next line of the quote you included, from the site you linked was:
More info: http://bit.ly/XLKKXb
That redirects you to a 2011 news article in Discovery.com called The Other Side of Otters, which includes
a witness account:
A weaned harbor seal pup was resting onshore when an untagged male sea otter approached it, grasped it with its teeth and forepaws, bit it on the nose, and flipped it over. The harbor seal moved toward the water with the sea otter following closely. Once in the water, the sea otter gripped the harbor seal’s head with its forepaws and repeatedly bit it on the nose, causing a deep laceration. The sea otter and pup rolled violently in the water for approximately 15 min, while the pup struggled to free itself from the sea otter’s grasp. Finally, the sea otter positioned itself dorsal to the pup’s smaller body while grasping it by the head and holding it underwater in a position typical of mating sea otters. As the sea otter thrust his pelvis, his penis was extruded and intromission was observed. At 105 min into the encounter, the sea otter released the pup, now dead, and began grooming.
The new article explains this was just one of at least nineteen occurrences documented in an article in the journal Aquatic Mammals.
A quick search in Google Scholar produces the actual article. Note: contains photographs that some people may find distressing.
- Heather S. Harris, Stori C. Oates, Michelle M. Staedler, M. Tim Tinker, David A. Jessup, James T. Harvey, and Melissa A. Miller, Lesions and Behavior Associated with Forced Copulation of
Juvenile Pacific Harbor Seals (Phoca vitulina richardsi) by
Southern Sea Otters (Enhydra lutris nereis) Aquatic Mammals 2010, 36(4), 331-341, DOI 10.1578/AM.36.4.2010.331
In conclusion: Yes, it happens. There are detailed descriptions documented by marine biologists, including photos.