Sadly, there’s no such thing as a perfect vacation and things can go just as wrong at sea as they do on land. Minor quibbles can be brought to the attention of the crewmembers you come in contact with on a regular basis. However, for the really big problems, you should work your way up the managerial ladder to the top until you are satisfied.
The most important thing to remember is that you should deal with problem situations on the ship when they occur; there's not much that can be done after the cruise is over.
What happens if you weren't able to resolve a situation on board? When it comes to post-cruise problem solving, Cruise Diva has suggestions for Effective Complaining.