Ramblings of an itinerant rocket scientist...

Yes, hornpipes are nautical. They are an Irish dance form associated with the Navy. The distinguishing characteristic in this case is that there are several places in the dance where one is up on one's toes with one foot right in front of the other, and then rocks one's ankles back and forth side to side in time with the music, while maintaining that position. It requires quite a bit of ankle and calf strength to do it. I learned to do these at the Starry Plough, a pub in Berkeley where they have Irish dancing every Monday night.