[Spock lowered his head and looked over his shoulder... he was slightly confused... why would the Captain ever want to dance with him?]

It is not illogical. Vulcans do dance... though it does not happen often. However, as the son of an ambassador it was required that I learn the different actions required at social events... dancing included. I have been trained in most formal styles of dance...

[Spock turns around so that he is fully facing Jim... his back to the railing.
He tilts his head...trying to figure out the situation.]