Give the captain heads up on itinerary of your plans and if desired you may invite him ("we are taking tour of VG on Thursday and you are invited if you would like to come or you if you have things you would rather do, we can meet you back at the boat sometime before 8am Friday" ... He may want to visit friends/ family. If eating ashore, suggest inviting at least one meal. If he is "on his own", no need to give him cash at this point, but gratuities customary at end of the trip.