A captain holds a state diploma (Capitaine 200 or Yacht Master for instance). They are paid for their activity.
During your cruise, they are the one at the helm !
An owner on board is generally the owner of the boat who whises to remain on board as a passenger. They cannot be paid for their activity.
The owner can share with you their knowledge of the boat and the navigation area. However, they cannot touch the controls, even for maneuvers.