Tipping practices are not universal, so a discussion such as this might be instructive. Unfortunately these debates trend towards the discussion of what is fair and what isn't, and whose "system" is the best. That is all academic. In this case, Jayne from England has brought up an issue that needs to be resolved within the context of the system of payment as it currently exists.
Passengers are expected to pay the crew in the form of gratuities rather than through a direct charge added to the initial fare. Without tips, the crewmembers involved are little more than indentured servants, working for room and board only. Therefore, going on the cruise with the intention of not tipping at all is unfair to the crew who will be serving you. Whether or not you decide to tip is up to you, Jayne. Now that you understand the system as it is, it is up to you to decide what you believe is the best course of action.