Dietary requests on a cruise ship

A lot of people worry about their dietary requirements on a cruise ship.
I don't just mean worrying about what the food will be like but about specific requests like Halal or Kosher food. You may be vegetarian or a vegan; you may have a lactose or wheat intolerance; you may even be deathly allergic to certain food groups.
Well you don't need to worry, comparing it to a land based holiday nothing could be easier to manage for you.