Not to turn this thread into a more general discussion of tipping, but if the purpose of the tip is to reward good service, how would the crew know whether someone who left a tip of a certain amount X is an American who thought the service was sub-par or a Belgian who thought it was the best he had ever received on a liveaboard? I think tipping is a relatively ineffective way to provide feedback. If one is going to leave a small tip for so-so services, at least follow up with the liveaboard management and explain what you thought they could have done better.
"Bad service" should result in a lot of negative reviews posted on review sites, and the liveaboard suffering from lack of bookings as a result. "Good service" should result in the crew being paid well relative to other liveaboards that get less stellar reviews and keeping their jobs. Liveaboards with really stellar service usually charge more in the first place, so one might reasonably believe that premium for good service is already baked into the price.