JJK2008, thank you. I have often thought the same.
Encouraging people to tip starts the slippery slope to expected tipping.
Life in general, not specific to cruise ships
At most, if anything - round up taxis and pizza deliveries to nearest round number - ie $47:50 - make it $50. Keep the change sort of thing.
and if somebody really does exceptional service, not just does their usual job - publicly say so - on Trip advisor,their Facebook page, write letter to their management,send flowers to their workplace etc