A bad behavior spree by a sea otter that took indecent liberties and actually killed up to 20 young seals a few years back seems more extreme than the much-noted recent example of a a seal that attempted to mate with a penguin.
Somehow, the heinous otter crimes have gone unnoticed. The story is that at least two sea otters sexually assaulted young seals and killed the victims through drowning or shock. Most of the problem was caused by a certain sea otter, dubbed "Morgan," that had been found abandoned as a baby and rehabilitated by people and later relased into the wild. But at least one wild sea otter was seen doing similar bad things. The end of the story? Morgan the very bad otter was captured.
Scientists and commenters have suggested that the recent 40 minute seal attack on the penguin is "the most unusual case of mammal mating behavior yet known" but I don't think so. A serial rape and murder spree by sea otters victimizing young seals seems more unusual.
Sea otters certainly look cuddly, but looks can be deceiving.