As impressive as this is, it's not that surprising, and I don't even think it's accurate to call it a 6th sense. Some animals have some senses that are much stronger than ours, and they've adapted to respond to them instinctively (or just get spooked enough by some things to be useful).
For example, elephants are known to have extremely good hearing, and I believe that it's particularly sensitive in the ultra-low-pitched area, below anything humans can hear. As such, earthquake "noises" (and quite possibly strange ocean sounds) could be something they're more likely to hear than a human, and respond accordingly by running away from them.
We have our own well-tuned senses, of course, though we rarely pay attention to them--a dog might have such a good sense of smell that it can tell when a bear is a quarter mile away, but it also usually has the good sense to run away from it. We can see well enough to notice the ocean receed hundreds of feet, but instead of getting spooked often go down to check it out. We can also see the subtle color differences in the sky that indicate a forest fire miles away or an unnatural concentration of NOx above a city skyline, as well, but again we rarely do anything with that information.
