A letter to the hotel director naming exceptional staff is communicated forward to those named and can positively influence their futures.
More immediately, Silversea operates a Crew Welfare Fund into which passengers can donate a cash sum which then provides amenities for all the crew. So the engineer, laundryman, deckhand etc that would normally not be seen (but without whom the cruise couldn't operate) are rewarded.
Finally, you can, of course, reward individuals with personal tips.