We went on a cruise for my wife's aunt's 100th birthday. My wife and I have been on 18 cruises and I had never seen anyone celebrate a 100th birthday. I've seen several 50th Anniversary couples celebrating and the cruise lines have always made a big deal out of the celebrating couple. You didn't say which cruise line you are going on, but of all the cruise lines I've been on, Princess made the biggest deal about celebrating anniversaries.
Be sure to tell the cruise director that it is their 50th anniversary. The cruise director recognized my wife's aunt's 100th birthday at one of the performances at night. She was invited to meet the captain and he ended up giving her a photo of him and her together and one with the family with the captain. (There were 9 of us. She brought along two of her younger sisters.)
The cruise director made all the arrangements. The Captain told my aunt she was the first 100 year old to celebrate their birthday on his ship and he had been captain for 20+ years. What they do for you depends on how rare a 50th anniversary is for him and his career and the ship.
The cruise line (before you go) and the cruise director (after you get on board) can give you lots of suggestions. I hope you have a wonderful time.
Suggestion: The contestants for Miss America have ribbons that have the name of their state on it when they wear fancy dresses. Have two made up for your parents. "Golden Anniversary 1962 - 2012" in black letters on a golden ribbon would look really impressive and they could wear it when a t-shirt wouldn't be appropriate. The other passengers would love it too and always congratulate your parents.
You can also give them an oversize button that says 5 kids, 18 grand kids 12 great grand kids or whatever their numbers are.
BTW, is this their first cruise?
Good luck to you and yours.