There's no unsurance that I know of which would cover any type of refund if you did get a Noro Virus on a ship.
And it's nothing that the cruise lines are doing to cause you to get ill. In fact they do whatever they possibly can to prevent it.
Since the beginning of the year one of the hospitals in our area has had over a dozen outbreaks of Noro Virus. The fact is you can get it anywhere, from other people. The key is washing your hands OFTEN.
If you want to be guranteed that you are not going to pick it on a ship, I'm afraid the only answer is not to go.