Possible Clues:

• Yes, that would sound less than human on board
• It's yes for the primate, and for 7 down
• Nocturnal primate - seaman's expression of compliance
• A lemur; yessir!
• Primate has two votes
• Yes, it's on board that you'll find the primate
• Yessir!; a lemur
• On board one is agreeable to have something subhuman
• How the primate is willing to be on board
• With never a hint of sarcasm, say, sea's yes?